Anti-Aggregation-Enabled Colorimetric Sensing Applications of Nanoparticles

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Muthaiah Shellaiah	--	17557	2022-12-27 09:07:52	\|
2	Anti-Aggregation-Enabled Colorimetric Sensing Applications of Nanoparticles	Muthaiah Shellaiah	-4839 word(s)	12718	2022-12-28 05:04:56	\| \|
3	format correct	Vivi Li	-3851 word(s)	8867	2022-12-29 07:43:20	\|

This entry is adapted from the peer-reviewed paper 10.3390/chemosensors10120536

Gold- and silver nanoparticles (Au NPs and Ag NPs)-based colorimetric detection of specific analytes has attracted intense research interest and is still in great demand. The majority of Au NPs- and Ag NPs-based sensory reports have revealed that, during the analyte recognition, dispersed NPs typically aggregated and displayed color changes from wine-red to blue/purple and yellow to orange/brown, respectively. On the other hand, only a few reports demonstrated that the aggregated Au NPs and Ag NPs underwent anti-aggregation in the presence of certain analytes, which displayed reversed color changes from blue/purple to wine-red and orange/brown to yellow, correspondingly. There are some examples of anti-aggregation phenomena mentioned in a vast number of studies on Au NPs- and Ag NPs-based colorimetric sensors via NP aggregation.

ion detection aggregation anti-aggregation Au NPs Ag NPs colorimetric recognition real analysis biosensors pesticides herbicides

1. Introduction

To sustain the eco-systems, detection and quantification of biological and environmental contaminants, such as metal ions, anions, bio-analytes, pesticides, and herbicides, are essential ^[1]^[2]^[3]^[4]^[5]. Thus, several tactics have been employed for analytes/hazards detection, including nanoprobes-based colorimetric/fluorometric assay, instrumentation-based quantification, metal-organic frameworks (MOFs)-tuned analyte detection, and organic/polymeric probes-based sensors, etc. ^[6]^[7]^[8]^[9]^[10]^[11]^[12]^[13]^[14]^[15]. Among them, the luminescent probes are the top performers in terms of their real-time bioimaging applications ^[16]^[17]^[18]^[19]^[20]. Likewise, the gold- and silver nanoparticles (Au NPs and Ag NPs)-based colorimetric assay of biological and environmental contaminants is attested to be effective in real water interrogations ^[21]^[22]^[23]^[24]^[25]. In the presence of a specific analyte, the Au NPs and Ag NPs displayed a color change from wine-red to blue/purple and yellow to orange/brown, respectively ^[26]^[27]^[28]^[29]^[30]. In these sensory investigations, the color change was attributed to the analyte-induced aggregation of dispersed Au NPs and Ag NPs ^[31].

In contrast, specific analytes can induce the anti-aggregation to reverse the origin color of Au NPs and Ag NPs from blue/purple to wine-red and orange/brown to yellow, correspondingly ^[32]^[33]. In fact, other than NPs-based colorimetric sensors, anti-aggregation is also a property possessed by many nanomaterials, such as selenium nanoparticles (Se NPs), cerium oxide (CeO₂), copper nanoparticles (Cu NPs), and molybdenum disulfide nanosheets (MoS₂ NS) and has been applied in amyloid-beta inhibition, reactive oxygen species (ROS) generation, disease treatment, etc. ^[34]^[35]^[36]^[37]^[38]. To achieve anti-aggregation-induced colorimetric responses, both aggregated Au NPs and Ag NPs must interact with specific aggregation-inducing agents or reactive analyte sensing units, which have a high sensitivity for specific analytes and can be released from the NPs’ surface ^[39]. In the presence of specific analytes, aggregation-inducing agents can be replaced by electrostatic interactions, disturbance of coordinative binding, oxidation/reduction of aggregation-inducing agents, and coordinative complexation with analyte, thereby re-dispersing the Au NPs/Ag NPs and reversing the color changes ^[32]^[33]^[34]^[35]^[36]^[37]^[38]^[39].

Highly concentrated toxic/heavy metal ions, anions, bio-analytes, pesticides, and herbicides significantly affect the eco-system ^[40]^[41]^[42]^[43]^[44]^[45]. Researchers developed numerous tactics including the anti-aggregation strategy to overcome contaminations caused by the above toxic/hazard species ^[6]^[7]^[8]^[9]^[10]^[11]^[12]^[13]^[14]^[15]^[16]^[17]^[18]^[19]^[20]^[21]^[22]^[23]^[24]^[25]^[26]^[27]^[28]^[29]^[30]^[31]^[32]^[33]. Recently, a review paper discussing the use of anti-aggregation of Au NPs for metal ions detection was reported by Najafzadeh et al. ^[46]. However, a thorough review on the anti-aggregation of Au NPs/Ag NPs for discriminating anions, bio-analytes, pesticides, herbicides, and metal ions detection is not yet available, which motivates researchers to write this entry.

In this entry, utilization of the anti-aggregation strategy in the Au NPs/Ag NPs-based colorimetric detection of metal ions, anions, bio-analytes, pesticides, and herbicides is illustrated with mechanistic and binding details. Uniqueness of its design, linear range, limit of detection (LOD), and real-time applicability are discussed with future scopes. The schematic shown in Figure 1 illustrates the anti-aggregation strategy applied in Au NPs/Ag NPs-based colorimetric sensors toward metal ions, anions, bio-analytes, pesticides, and herbicides.

Figure 1. Schematic of the anti-aggregation strategy applied in Au NPs/Ag NPs-based colorimetric sensors toward metal ions, anions, bio-analytes, pesticides, and herbicides.

2. Au NPs-Based Colorimetric Assay of Metal Ions via Anti-Aggregation

In regard to the anti-aggregation-enabled Au NPs-based colorimetric metal ions detection, quantification of Hg²⁺ has attracted much attention by numerous researchers ^[47]^[48]^[49]^[50]^[51]^[52]^[53]^[54]^[55]^[56]^[57]. Li and co-workers described the use of label-free citrate–capped Au NPs (CI-Au NPs; particle size = 13 nm; (nm = nanometer)) as a colorimetric probe toward mercury ions detection via the anti-aggregation strategy ^[47]. In this work, the 4,4′-dipyridyl (DPy; 5.2 µM (µM = micromole)) acted as an aggregation-inducing agent of Au NPs that can be released from the NPs’ surface because of its exceptional coordination ability toward Hg²⁺. When the 4,4′-dipyridyl induces Au NPs aggregation, the color changes from red to blue. However, in the presence of Hg²⁺, it changes back to its original red color due to anti-aggregation, as represented in Figure 2A. None of the metal ions, except Hg²⁺, displays colorimetric reversibility as seen in Figure 2B. The best result was obtained at pH 7 (buffer: 10 mM Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl)) after 30 min incubation (min = minutes). Transmission electron microscopy (TEM) studies clearly attested the anti-aggregation of Au NPs. Surface plasmon resonance (SPR at A₅₂₀/A₆₂₀) studies revealed the linearity to be between 0.04 and 1.1 µM, with an estimated limit of detection (LOD) of 15 nM (nM = nanomole). This work was also demonstrated with tap water and spring water samples (with 100-fold excess interference ions) investigations; therefore, it can be utilized for Hg²⁺ quantification in real time.

Figure 2. (A) Schematic representation of anti-aggregation of Au NPs for the detection of Hg²⁺; (B) absorbance ratio of A₅₂₀/A₆₂₀ and photographic images (inset) of solutions containing Au NPs, DPy (4,4′-Dipyridine; 2.5 µM), and various metal ions (each 100 µM, except Hg²⁺ 1.0 µM) ((A,B) are reproduced with the permission from ^[47]); (C) schematic illustration of the determination of Hg²⁺ by PDDA-Au NPs in the presence of Cys; (D) the SPR band shift relative to Cys-Au NPs (∆λ) and photographic image of Au NPs solutions containing 0.7 µg/mL Cys, 1.0 × 10⁻⁴ M EDTA-Na₂, and 1.0 × 10⁻⁵ M various metal ions. ((C,D) are reproduced with the permission from ^[48]).

Following a similar approach, Ding et al. described anti-aggregation-mediated colorimetric detection of Hg²⁺ by dislocating the aggregation-inducing agent cysteine (Cys; 7 µg/mL) at 0.1 M NaCl ^[48]. In the study, positively charged Au NPs were synthesized using poly(diallyldimethylammonium) chloride (PDDA) to afford PDDA-Au NPs (particle size = 12.2 nm). The PDDA acted as a reducer and stabilizer at the same time. Adding the Cys into PDDA-Au NPs led to a color change from ruby red to royal purple via aggregation. In the early stage, the aggregation was negligible due to the strong electrostatic repulsion between PDDA-Au NPs and Cys. However, after adding 0.1 M of NaCl, the electrostatic repulsion became weaker, and the Cys-Au NPs aggregates were formed. A pH value of 5 (Buffer: acetic acid–sodium acetate (HAc-NaAc)) was selected to enhance the strong Au-S binding (via zwitterionic Cys form) in the Cys-Au NPs aggregates. Owing to the strong interaction between Cys and Hg²⁺, it was more favorable for the Hg-Cys complex formation; therefore, it prevented the Cys-Au NPs aggregates and reversed the color back to ruby red. This anti-aggregation behavior was demonstrated by TEM and SPR (at A₅₂₀/A₆₁₀) studies. Among other metallic species, Pb²⁺ and Cu²⁺ also displayed a certain degree of anti-aggregation by forming a complex with Cys, which can be avoided by using the masking agent EDTA-Na₂ (ethylenediaminetetraacetic acid disodium salt). Figure 2C,D demonstrate the anti-aggregation of the Au NPs-Cys system in the presence of Hg²⁺. The Hg²⁺ shows the linear range to be between 0.05 and 10 µM with an LOD of 25 nM. This work was demonstrated in drinking water samples (>100% recovery), but more interrogations are required to further understand the underlying mechanism.

Aggregation of citrate-capped (CI) Au NPs (particle size = 13 nm; conc. = 15 nM (conc. = concentration)) was induced by thymine via citrate replacement from the NPs’ surface. When Hg²⁺ was present, it interacted with thymine to afford anti-aggregation-enabled reversibility of the original SPR and color ^[49]. In the presence of thymine (0.2 mM), the color of CI-Au NPs turned to blue. When Hg²⁺ was added, a “thymine-Hg²⁺-thymine” motif was formed, which released the thymine from the Au NPs’ surface and reversed the color back to the original red through the anti-aggregation (see the TEM and SPR (at A₅₂₀/A₆₇₀) interrogations). Optimum pH condition and incubation time were established as pH 8 (buffer: 1 mM Tris-HCl) and 30 min. None of the ions, except the Hg²⁺, induced anti-aggregation and exhibited significant recoveries (88–112.5%) in tap water samples. This report can be regarded as a novel work in terms of its uncomplicated procedure, linearity (2–12 µM), LOD (2 nM), and real-time applicability. Subsequently, O-phenylenediamine (OPD; 3.1 µM) induced CI-Au NPs aggregations (particle size = 7–11 nm) and displayed a color change from red to blue ^[50]. By adding Hg²⁺ to the OPD-Au NPs complex system, the anti-aggregation was initiated via OPD-Hg²⁺ complexes formation (after 20 min incubation time), and the color was changed back from blue to red. The effective coordination between Hg²⁺ and OPD is directed by two amine (-NH₂) groups present in OPD at an operative pH value of 6.5. As seen in Figure 3A,B, none of the metal ions, except Hg²⁺, shows high selectivity via anti-aggregation. This was also confirmed by TEM, SPR (at A₅₂₀/A₆₈₀), and density functional theory (DFT) investigations as well. The linear regression of Hg²⁺ was between 0.01 and 2 µM, with an LOD of 5 nM. This work was applied to tap water and lake water samples interrogations, which showed recoveries of between 70 and 120%. In terms of its high colorimetric selectivity, linear range, LOD, theoretical support, and real sample studies, this report can be regarded as a unique work in Hg²⁺ quantification.

Figure 3. Selectivity of the Au NPs-based detection system (containing 3.1 µM of OPD) for Hg²⁺ compared with other ions. (A) Photograph of the detection systems incubated with Hg²⁺ (2.0 µM) or other ions (20 µM); (B) A₅₂₀/A₆₈₀ (ratio of the absorbance value at 520 nm to that at 680 nm in the UV–vis spectra) of the detection systems incubated with Hg²⁺ (2.0 µM) or other ions (20 µM) (mean ± SD, n = 3) ((A,B) are reproduced with the permission from ^[50]).

The CI-Au NPs were engaged in anti-aggregation-enabled colorimetric detection of Hg²⁺ as described by the following: The L-penicillamine (Pen; optimum concentration fixed as 3.5 µM) induced aggregation of CI-Au NPs (particle size = 20 nm; conc. = 2.85 nM) as evident from the emerging of a new absorbance peak at 750 nm. The absorption peak was blue-shifted to 525 nm due to anti-aggregation and effective binding of penicillamine to Hg²⁺ (via Pen-Hg²⁺-Pen complex) when Hg²⁺ was added ^[51]. Both aggregation and anti-aggregation were attested by TEM studies. Detection of Hg²⁺ showed the linearity as being between 50 and 400 nM (at A₅₂₅/A₇₅₀), with an LOD of 25 nM. This work was supported by spiked tap water investigations, but the appropriate pH for this sensor was predicted to be acidic pH 2.4 (buffer: 5 mM citric acid–sodium citrate), thereby requiring more clarifications on its reliability. Zhou et al. utilized the 4-mercaptophenylboronic acid (MPBA; 10 µM) as an aggregation-inducing agent with CI-Au NPs (particle size = 15 nm) for colorimetric detection of Hg²⁺ by means of anti-aggregation ^[52]. The citrate attached on the NP surface was replaced by MPBA (via Au-S bond) and led to aggregation, thereby changing the color from red to blue.

When Hg²⁺ is present, the MPBA coordinates with Hg²⁺ via strong Hg-S interactions to initiate the anti-aggregation accompanied with a color change from blue to red as demonstrated in Figure 4A. The best result was attained at pH 4 (buffer: 10 mM citric acid–sodium citrate) after 20 min incubation.

Figure 4. (A) Schematic presentation of the Au NPs colorimetric mechanism for Hg²⁺ detection; (B) photographic image; (C) UV–vis absorbance spectra and (D) linear plot of the A₅₂₀/A₆₉₀ vs. Hg²⁺ concentration (c) of the MPBA (10 µM)-Au NPs solution (pH 4.0) containing various concentrations of Hg²⁺: (1) 0, (2) 0.01, (3) 0.05, (4) 0.1, (5) 0.2, (6) 0.8, (7) 1.2, (8) 2, (9) 3, (10) 4, (11) 5, (12) 6, (13) 8, and (14) 10 µM. ((A–D) are reproduced with the permission from ^[52]).

Detection of Hg²⁺ shows colorimetric response and linear regression as being between 0.01 and 5 µM (at A₅₂₀/A₆₉₀), with an LOD of 8 nM, as seen in Figure 4C,D. TEM studies confirmed the anti-aggregation in the Hg²⁺ detection, and none of the other metal ions displayed the interfering effect. Moreover, spiked lake water interrogations showed recoveries of between 96.3 and 110%, thereby approving its real-time applicability. In a similar fashion, free thiol (-SH) containing aggregation-inducing agents, such as N-Acetyl-L-cysteine (NAC; 10 µM; 30 min incubation time; pH 7 (buffer: phosphate buffer saline (PBS) prepared from NaH₂PO₄ and Na₂HPO₄)) and 2-Mercapto-benzothiazole (MBT; 2.2 µM; 5 min incubation time; pH 5.8 (buffer: PBS)), were used in discrimination of Hg²⁺ ^[53]^[54]. Both agents induced aggregation of CI-Au NPs (particle size = 15 nm (conc. = 15 nM) and 10–20 nm, respectively) and observed a color change from red to blue, which was then disrupted by Hg²⁺-enabled anti-aggregation with a reversed color change from blue to red. The dimeric complexes (NAC-Hg-NAC and MBT-Hg-MBT) were formed with those reactive agents because of the high binding tendency of Hg to S. The linear regression of Hg²⁺ was reported to have values between 0.02 and 1 µM (at A₅₂₀/A₆₂₀) and 0.05–1 µM (at A₇₀₀/A₅₂₀), with calculated LODs of 9.9 nM and 6 nM, respectively. Both reports were well authorized by TEM and recovery studies in real water and milk samples analysis; therefore, they can be regarded as notable reports in anti-aggregation-mediated Hg²⁺ quantification.

Rajeshwari and co-workers proposed a tween 20-modified gold nanorods (Au NRs; average length = 45.11 ± 0.64 nm and diameter = 17.95 ± 0.38 nm)-based detection of Hg²⁺ via the anti-aggregation strategy ^[55]. Herein, dithiothreitol (DTT; 1.5 µM) firstly induced aggregation of NRs. When Hg²⁺ was added, the strong binding affinity of DTT to Hg²⁺ turned the aggregations into dispersed NRs. The linear regression of Hg²⁺ was between 1 and 100 pM (SPR at 679 nm; pH 7.4 (buffer: Tris-HCl); pM = picomole), with an LOD of 0.42 pM. This work also conducted studies in stimulated body fluid recovery, but they lacked colorimetric responses. Sun et al. utilized the hexadecyl trimethyl ammonium bromide (CTAB; 0.44 µM) to induce aggregation of CI-Au NPs (particle size = 13 nm; conc. = 8.7 nM), which was dis-aggregated by Hg²⁺ via anti-aggregation with a corresponding color change from blue to red ^[56]. The best pH and response time were determined to be pH 7 (buffer: Tris-HCl) and 30 min. In the beginning, aggregation of NPs took place due to strong electrostatic attraction between CI-Au NPs and CTAB, with a respective color change from red to blue. In the presence of Hg²⁺, Au NPs tend to form a Hg-Au alloy, which hinders the electrostatic attraction and inhibits aggregation caused by CTAB, as shown in Figure 5A. The linear range of Hg²⁺ was between 0 and 0.68 µM (at A₆₅₀/A₅₂₀), with an LOD of 11.9 nM. The anti-aggregation was confirmed and attested by TEM and DLS (DLS = dynamic light scattering) investigations and by tap water recovery (>90%; RSD ≤ 4% (RSD = relative standard deviation)). Therefore, it can be regarded as an important probe toward Hg²⁺ quantitation. Thereafter, 3, 5-Dimethyl-1-thiocarboxamidepyrazole (Pzl; 1.5 µM)-induced aggregation of CI-Au NPs (particle size = 14 nm; conc. = 15 nM) was utilized for the colorimetric detection of Hg²⁺ ^[57]. In the presence of Pzl, the color of the CI-Au NPs changed from red to blue via aggregation, which was reversed (from blue to red) in the presence of Hg²⁺ by means of anti-aggregation. The reason of the color reverse to the original (as shown in Figure 5B) is that the Pzl goes through coordination with Hg²⁺ to form a Pzl-Hg-Pzl complex due to the presence of the functional unit -C = S. None of the ions, except Hg²⁺, displayed interfering selectivity. The optimum response was found at pH 5.6–8 (buffer: PBS) after 3–4 min incubation. The anti-aggregation mechanism and dimeric mercury complex of Pzl were clarified and confirmed by TEM studies and DFT optimization, respectively. The linearity of Hg²⁺ was between 0.01 and 1.5 µM (at A₆₈₀/A₅₂₀), with an LOD of 7.7 nM, and was attested by recovery studies in tap water and lake water (lies between 94 and 105%). Based on the TEM, DLS, and real samples analysis, this report can be regarded as a well accomplished work.

Figure 5. (A) The sensing mechanism of the colorimetric sensor based on anti-aggregation of Au NPs for Hg²⁺ detection (reproduced with the permission from ^[56]) and (B) schematic representation of the 3, 5-dimethyl-1-thiocarboxamidepyrazole (Pzl)-derived colorimetric detection mechanism for Hg²⁺ (reproduced with the permission from ^[57]).

Other than the Hg²⁺ detection, anti-aggregation-mediated discrimination of Ag⁺ was also demonstrated with CI-Au NPs, as described next. Mao et al. described CI-Au NPs’ (particle size = 13 nm) anti-aggregation-tuned selective detection of Ag⁺ ^[58]. Herein, the thiamazole (2 µM) was used as an aggregation-inducing agent to produce an Au NPs color change from red to blue. The aggregation was then inhibited in the presence of Ag⁺. Due to Ag⁺-enhanced oxidization of sulfhydryl groups (-SH), anti-aggregation takes place to reverse the color from blue to red, as illustrated in Figure 6A. TEM study confirmed the involvement of the anti-aggregation strategy with the optimum pH value and response time fixed at 7.5 (buffer: Britton Robison (BR) buffer solution made up from H₃PO₄, H₃BO₃, and HAc (0.04 M)) and 15 min. The dynamic linear range of Ag⁺ was between 0.1 nM and 90 μM (at A₅₂₀/A₆₆₀), with an LOD of 0.042 nM. Note that there was no interfering effect in the presence of other ions, and real water (tap and river) samples displayed recoveries of >98%. This is a nice work (toward Ag⁺ quantification) in terms of its performance in the linear range, LODs, and real-time applicability.

Figure 6. (A) Schematic representation of the analytical process for Ag⁺ detection (reproduced with the permission from ^[58]); (B) schematic representation of the Au NPs-based colorimetric assay for silver; (C) color changes with increasing concentration of Ag⁺ from 0 to 9 µM; (D) SPR absorption change of Au NPs in tris solution with Ag⁺ concentration varied from 0 to 9 µM; (E) the linear response of the system to Ag⁺ ion concentration. (The concentration of tris solution was 9 mM, and the pH was fixed at 7.5. Au NPs’ concentration was 1.2 nM.) ((B–E) are reproduced with the permission from ^[59]).

Label-free Au NPs (particle size = 12 nm; synthesized via NaBH₄ reduction; SPR at 512 nm; conc. = 20 nM) composited with ascorbic acid (AA) were engaged in Ag⁺ detection without employing any aggregation agent ^[60]. The AA-tuned reduction of Ag⁺ resulted in Ag⁰ on Au NPs’ surface and formed an Ag–Au alloy. The linearity of Ag⁺ was between 2 and 28 µM (SPR at 385 nm), with a calculated LOD of 0.85 µM. Even though this work was demonstrated in drinking water samples, it cannot be categorized as an anti-aggregation strategy-based sensor. Colorimetric detection of Ag⁺ based on anti-aggregation of CI-Au NPs (particle size = 20 nm; conc. = 1.2 nM) was proposed by Safavi and co-workers ^[59], wherein tris(hydroxymethyl) aminomethane (tris at 9 mM; pH 7.5) was used as the aggregation-inducing agent to deliver a color change from red to blue, with a new SPR peak occurring at 650 nm. The anti-aggregation-enabled colorimetric response is observed with a color change from blue to red (within 20 min) due to the strong interaction between tris and Ag⁺, as represented in Figure 6B. The Ag⁺-mediated anti-aggregation was well established from TEM, SPR, and interference studies. The color change (from blue to red), SPR changes, linear range (1–9 µM; at A₅₂₀/A₆₅₀), and LOD (0.42 µM) calculations are shown in Figure 6C–E. This work was demonstrated by wastewater and river water investigations with a recovery >96%; therefore, it can be regarded as a nice innovation.

Gao et al. described the anti-aggregation-facilitated detection of Cu²⁺ using CI-Au NPs (particle size = 18 nm; conc. = 3.89 nM) with glutathione (GSH; 5 µM) as the aggregation-inducing agent ^[61]. The free thiol (-SH) of GSH replaced the citrate from the Au NPs’ surface to induce the aggregation in a short time and produced a color change from red to blue with a corresponding SPR shift from 520 nm to 650 nm. In the presence of Cu²⁺, the GSH was oxidized to form GSSG, and the anti-aggregation took place to reverse the color from blue to red, with a shifting of the SPR peak from 650 nm to 520 nm. The best result was obtained at pH 3.4 (buffer: 4 mM BR) after 15 min incubation. It displayed a linear regression of between 20 and 500 nM (at A₆₅₀/A₅₂₀), with an LOD of 20 nM with Cu²⁺, and was also demonstrated by tap water recovery (>96%; RSD ≤ 3%). Moreover, colorimetric reversibility of Cu²⁺ with S²⁻ was also discussed in this report. Therefore, based on TEM, colorimetric/SPR responses, interference, kinetic, temperature, and reversibility investigations, this report can be attested as an exceptional work in Cu²⁺ detection. Thereafter, D-penicillamine (D-PC; 30 µM)-enabled aggregation of CI-Au NPs (particle size = 13 nm; conc. = 15 nM) was demonstrated in detecting Cu²⁺ via the anti-aggregation strategy ^[62]. The D-PC induced the aggregation via providing its free -SH unit to CI-Au NPs, which resulted in a color change from red to blue. After adding Cu²⁺, D-PC coordinated with Cu²⁺ to form dimeric complexes (D-PC-Cu-D-PC via the S-Cu-S bond) and enabled anti-aggregation to reverse the color change from blue to red. The best result was found at pH 7 (buffer: 1 M NaCl-10 mM NaOH) in the presence of 25 mM NaCl and after 12 min incubation. The linear range of Cu²⁺ was between 0.05 and 1.85 µM (at A₅₂₀/A₆₅₀), with an LOD of 30 nM. TEM and SPR studies confirmed the anti-aggregation. The real water (tap water, cascade water, and mineral water) interrogations showed recoveries of between 85 and 108% (RSD = 1.43–6.93%). Thus, this work can be adopted in real time detection of Cu²⁺; however, careful optimizations on the concentrations of D-PC, NaCl, and pH are required.

In contrast to the aforementioned metal ion sensors in this section, Deng and co-workers described the Schiff base and Lewis acid–base interaction-regulated anti-aggregation for colorimetric detection of Sc³⁺ ^[63], wherein the biomolecule pyridoxal phosphate (PLP; 5 µM; contains free -CHO group) reacted with cysteamine-functionalized Au NPs (Cyst-GNPs; particle size = 17.5 ± 3.4 nm (n = 100) positively charged with free -NH₃⁺ at pH 6) to form a neutral Schiff base and induced aggregation to produce a color change from red to blue. The competition between the intense Lewis acid–base interactions of Sc³⁺ with PLP and Cyst-GNPs binding to PLP enables the anti-aggregation of Cyst-GNPs to display a reversed color change from blue to red, as shown in Figure 7. TEM, DLS, and SPR responses, and interference studies confirmed the anti-aggregation-based high selectivity of Sc³⁺. The incubation time of all experiments was established as 1 min at pH 6 (buffer: HAc-NaAc). The linear regression of Sc³⁺ was between 0.1 and 3 µM (at A₇₀₀/A₅₂₅), with an estimated LOD of 20 nM. The tap water recoveries of Sc³⁺ in this method were between 96.87 and 101.8%, with corresponding RSD values of 0.6–3.6%. This is the only report in the anti-aggregation-mediated colorimetric detection of rare earth Sc³⁺; therefore, it can be noted as an exceptional work.

Figure 7. Schematic of PLP-triggered aggregation of Cyst-GNPs for colorimetric detection of Sc³⁺ (reproduced with the permission from ^[63]).

3. Au NPs-Based Colorimetric Quantification of Anions via Anti-Aggregation

Plaisen and co-workers described the detection of bromide anion (Br⁻) via the anti-aggregation strategy ^[64] as follows: CI-Au NPs (particle size = 16.43 ± 0.81 nm; conc. = 5.52 nM) aggregation took place in the presence of Cr³⁺ ions (optimum conc. = 4.81 µM), which resulted in a color change from red to blue. However, when Br⁻ was pre-mixed (at pH 6.5 (buffer: 10 mM sodium phosphate (Na₃PO₄)) and 10 min reaction time), the color overturned from blue to red via anti-aggregation. TEM and DLS studies confirmed the involvement of the anti-aggregation strategy. In fact, Br⁻ tends to affect the citrate capping via self-deposition over the NPs’ surface, which also hinders the binding of citrate to Cr³⁺, thereby resulting in the color change. The linear regression of Br⁻ was found between to be 0.31 and 3.75 µM (at A₅₁₉/A₆₇₃), with an LOD of 40 nM and a limit of quantification (LOQ) of 0.13 µM. This work was applied in rice sample investigations; thus, it can be regarded as a nice innovation.

Due to the strong binding affinity of Hg²⁺ to the iodide anion (I⁻), there are experiments and reports in using Hg²⁺ as an aggregation-inducing agent to conduct anti-aggregation-based colorimetric detection of I⁻. The N-1-(2-mercaptoethyl) thymine-capped Au NPs (particle size = 23 nm; conc. = 1.4 nM), T-rich single-stranded DNA (DNA = deoxyribonucleic acid)-capped Au NPs, (T-rich ssDNA–Au NPs; particle size = 13; conc. = 3 nM), and 2-hydroxyethyldithiocarbamate-capped Au NPs (2heDTC-Au NPs; particle size = 12.92 ± 1.44 nm; conc. = 15 nM) were proposed in Hg²⁺-enabled aggregation to engage in I⁻ detection via the anti-aggregation tactic ^[65]^[66]^[67]. The mercapto-functionalized thymine and T-rich ssDNA over Au NPs aggregated strongly in the presence of Hg²⁺ (300 nM and 2 µM, correspondingly) and induced color changes from red/wine-red to blue/violet-blue ^[65]^[66]. In fact, the aggregation is highly effective due to the strong attraction between thymine and Hg²⁺ via the T-Hg²⁺-T complex. Upon pre-treating I⁻ with Hg²⁺, the T-Hg²⁺-T complexation was interrupted by the HgI₂ formation, thus reversing the colorimetric response from blue/violet-blue to red/wine-red via anti-aggregation. The best pH condition in both reports ^[65]^[66] was established as pH 7.4 (buffer: PBS and 20 mM Tris-HCl (contains 20 mM NaCl before addition), respectively) with a distinct incubation time of 5 min and 10 min, respectively. Figure 8A demonstrates the Hg²⁺-induced aggregation and I⁻-enabled anti-aggregation of N-1-(2-mercaptoethyl) thymine-capped Au NPs. TEM, DLS, and SPR analysis attested to both aggregation (in the presence of Hg²⁺) and anti-aggregation by I⁻. Linear ranges of I⁻ by mercapto-functionalized thymine and T-rich ssDNA on Au NPs were determined as 10–600 nM (at A₅₂₄/A₆₄₅) and 40 nM–4 µM (at A₅₂₀/A₆₅₀), with LODs of 10 nM and 13 nM, respectively. Both reports show high selectivity and exceptional recoveries (>89%) in real water and human urine samples; thus, they can be regarded as good works.

Figure 8. (A) Schematic illustration of the analytical process for detecting iodide based on the anti-aggregation of N-1-(2-mercaptoethyl) thymine-capped Au NPs (reproduced with the permission from ^[65]) and (B) schematic illustration of the sensing mechanism based on the arginine-capped Au NPs and Cu²⁺ (reproduced with the permission from ^[68]).

Subsequently, the 2heDTC-Au NPs also aggregated and induced a color change from red to blue in the presence of Hg²⁺. The aggregation was interrupted in the presence of I⁻ with a reverse color change from blue to red via anti-aggregation ^[67]. The citrate on Au NPs were firstly modified via ligand exchange with 2-hydroxyethyldithiocarbamate (2heDTC), and then the interaction of Hg²⁺ with 2heDTC resulted in aggregated bare Au NPs. When I⁻ was added, the interaction of 2heDTC to Hg²⁺ was inhibited, and anti-aggregation occurred as supported by TEM, DLS, and SPR studies. The optimum pH and incubation time for this sensory study were fixed as pH 5 (buffer: 50 mM PBS) and 7 min. The linear regression of I⁻ was between 0.1 and 4 µM (at A₅₂₀/A₆₅₀), with an LOD of 20 nM. This work showed recoveries of >85% in edible salts; thus, it can be regarded as a unique colorimetric sensor for I⁻ quantification.

In this track, the arginine-capped Au NPs (Arg-Au NPs; particle size = 13 nm) were utilized for detecting I⁻ via anti-aggregation by engaging Cu²⁺ (70 µM) as the aggregation-inducing agent ^[68]. In the presence of Cu²⁺, the Arg-Au NPs began to aggregate with a colorimetric response from red to blue via Arg-Cu²⁺-Arg complexation. The dimeric Arg-Cu²⁺ complex was disturbed by adding I⁻ to induce the anti-aggregation and produce a colorimetric response from blue to red. The actual reaction between Cu²⁺ and I⁻ is represented by Equation (1) and schematically illustrated in Figure 8B:

2Cu²⁺ + 4I⁻ = 2CuI + I₂(1)

The best pH condition and incubation time were established as pH 7.4 (buffer: PBS) and 5 min. TEM, DLS, and SPR analysis also confirmed involvement of the anti-aggregation. The linear range of I⁻ was between 0.01 and 4 µM (at A₅₂₄/A₆₅₄), with an LOD of 10 nM. Drinking water and table salt investigations showed recoveries of between 96 and 108% and 94 and 103%, respectively, with RSD values of <3.9%. Though this work was effective in I⁻ quantitation, further optimizations on the Cu²⁺ concentrations are still required in future work. By means of anti-aggregation, CI-Au NPs were engaged for the colorimetric detection of I₂ using thiosulfate (S₂O₃²⁻; 5.94 mM) as an aggregation-inducing agent ^[69]. Initially, Au NPs react with S₂O₃²⁻ to form a strong Au-S bond and to induce aggregation of NPs (with a color change from red to blue). Then a redox reaction takes place toward the formation of Au(S₂O₃)₂³⁻, as described below in Equation (2):

4Au + O₂ + 2H₂O + 8 S₂O₃²⁻ → 4[Au(S₂O₃)₂]³⁻ + 4 OH⁻(2)

Upon pre-incubating I₂ with Au NPs, the free thiosulfate is produced due to another redox reaction between I₂ and S₂O₃²⁻, as described in Equation (3). Thereafter, anti-aggregation takes place to induce the color from blue to red.

I₂ + S₂O₃²⁻_(aq) → 2I⁻_(aq) + S₄O₆²⁻_(aq)(3)

TEM and SPR interrogations well attested the anti-aggregation strategy. The best result was achieved at pH 7 after 16 min incubation. The linear range of I₂ was between 3 and 80 nM (at A₅₁₉/A₆₄₀), with an LOD of 1.36 nM. Moreover, plasma and tap water investigations showed >97% recovery. However, this work still needs more experimental data on the particle size variations, DLS, and interference studies.

Deng et al. described the anti-aggregation-enabled detection of thiocyanate anion (SCN⁻) by engaging CI-Au NPs (particle size = 13 nm; conc. = 3.2 nM) and 8 mM of sulfuric acid (H₂SO₄) as the aggregation-inducing agent ^[70]. In the presence of H₂SO₄ (8 mM), the Au NPs aggregated with a simultaneous color variation from red to blue. When SCN⁻ was added, anti-aggregation (via forming Au-S bonds) was induced to produce a blue-to-red color change, which was validated by SPR and TEM studies. The SCN⁻ displayed a linear regression of between 0.25 and 2 µM (at A₇₀₀/A₅₂₀), with an LOD of 0.14 µM. Intensity of the color was visible to naked eyes at 1 µM concentration. The tap water recoveries ranged between 94.8 and 101.3%, with RSDs of 1.6–3.2%. It should be noted that this work validated the selectivity of SCN⁻ over a wide range of cations and anions. However, further study is mandatory to improve the LOD toward nM or PM. Moreover, its applicability requires justification due to the use of an acidic aggregation-inducing agent. Song and co-workers described the anti-aggregation-enabled detection of SCN⁻ by employing CI-Au NPs (particle size = 13 nm) and cetyltrimethyl ammonium bromide (CTAB; 0.5 µM) as the aggregation-inducing agent ^[71]. Due to the strong electrostatic interaction between CTAB and stabilizing agent citrate, the aggregation induced a color change from red to blue. When SCN⁻ was added, it strongly coordinated with the Au surface with a high stability constant; thus, Au NPs were re-dispersed via anti-aggregation and displayed a color change from blue to red. The SPR, color change, and nanoparticles size variations during subsequent interaction of CTAB and SCN⁻ are displayed in Figure 9A,B.

Figure 9. (A) UV−vis spectra and colors of Au NPs in the absence (a) and presence of 0.5 µM CTAB (b), after the addition of 0.5 µM CTAB and 1.5 µM SCN⁻ (c). (B) TEM images of Au NPs, naked Au NPs (a), and Au NPs with 0.5 µM CTAB: in the presence of 0 (b), 1.5 µM (c), and 2.0 µM SCN⁻ (d); scale bars for (a–d) = 200 nm ((A,B) are reproduced with the permission from ^[71]).

The probe had a high selectivity over a wide range of analytes and showed the best sensory performance to SCN⁻ at pH 7 (buffer: 10 mM PBS) after 15 min incubation. The linearity of SCN⁻ ranged between 0.1 and 1.5 µM (at A₆₅₀/A₅₂₀), with an LOD of 6.5 nM. Real studies on solid and liquid milk samples demonstrated a recovery of >90% with an RSD of ≤5.5%. Based on the selectivity, sensitivity, and real samples investigations, this work can be regarded as nice invention for SCN⁻ monitoring in real-time. By employing the 2-amino pyridine (2-AP; 50 µM) as the aggregation-inducing agent, CI-Au NPs (particle size = 10–44 nm; conc. = 8.3 nM) were engaged in detecting SCN⁻ ^[72]. In this work, the +vely charged 2-AP interacted with -vely charged CI-Au NPs to form the static electricity and to induce the aggregation with a color change from red to blue. Due to the strong binding affinity of SCN⁻ to Au NPs (via Au-S bond), the 2-AP-induced aggregation was disturbed, which resulted in the dispersion of Au NPs via anti-aggregation as evident from a color change from blue to red and SPR, DLS, and TEM studies. The optimum anti-aggregation induced by SCN⁻ occurred at pH 7 (buffer: 40 mM Tris-HCl) and displayed a linearity of between 0.4 and 1.2 μM (at A₆₇₅/A₅₂₀), with an LOD of 0.37 µM. Tap water investigations demonstrated recoveries of between 98.52 and 105.48%, with an RSD of <4.6%. Though the sensitivity of this work is high, further optimization is required to tune the LOD toward the nM level.

Lu et al. described the use of dithiothreitol (DTT; 4 µM) as an aggregation-inducing agent with CI-Au NPs (particle size = 13 nm; 11 nM) to quantify the hypochlorite (OCl⁻) anion via the anti-aggregation strategy ^[73], wherein the DTT with a free thiol unit at both ends enabled aggregation of Au NPs and displayed a color change from red to blue. Upon adding OCl⁻, a redox reaction takes place to oxidize the DTT (6-electron transfer process) to its sulfonate derivatives, which results in anti-aggregation of Au NPs with a color change from blue to red, as depicted in Figure 10A. The optimum condition for colorimetric detection of OCl⁻ was at pH 7.17 (buffer: 45 mM PBS), with incubation time of 2 min. Photographs and SPR, TEM, and XPS (XPS = X-ray photoelectron spectroscopy) studies attested to the anti-aggregation being induced by OCl⁻. With the OCl⁻ concentrations between 0 and 7 µM, the SPR peak changes at A₆₆₅/A₅₂₀, with an LOD of 2 µM, could be visualized with naked eyes. Figure 10B,C display the SPR and colorimetric changes as a function of the OCl⁻ concentration (0–8 µM). It should be noted that none of the interfering species could affect the OCl⁻ selectivity. Moreover, real water (tap water/lake water) investigations displayed a recovery >80%. However, this work lacks UV-based LOD calculations and DLS interrogations.

Figure 10. (A) Schematic representation of the hypochlorite sensor with unmodified gold nanoparticles; (B) optical absorption spectra in the presence of different concentrations of hypochlorite. Conditions: PBS (10 mM, pH 7.17), DTT (4.0 µM), and (C) photographs showing the colorimetric responses (the hypochlorite concentrations, in µM, are listed on the centrifuge tubes) ((A–C) are reproduced with the permission from ^[73]).

Liu and co-workers synthesized the mercapto–acetic acid–capped Au NPs (MA-Au NPs; particle size = 13 nm; conc. = 3 nM) and were employed in the detection of phosphate (PO₄³⁻; Pi) anions via the anti-aggregation tactic ^[74] in which the Eu³⁺ (20 µM) was used as the aggregation-inducing agent. Adding the Eu³⁺ to MA-Au NPs induced aggregation and a color change (from red to blue) because of the effective coordination with free carboxyl (-COOH) units of mercapto–acetic acid. Pre-mixing of Pi anion into the above mixed system inhibits formation of -COOH-Eu³⁺ complex via strong Pi-Eu³⁺ binding, as seen in Figure 11. TEM and SPR studies confirmed the anti-aggregation, and the best pH and incubation time were determined as pH 7.4 (buffer: 25 mM Tris-HCl) and 5 min. The linear range of Pi was between 0.5 and 30 µM (at A₅₂₀/A₆₇₀), with an LOD of 76 nM. Real samples studies demonstrated selectivity over other interferences but lacked spiked recovery investigations. However, this is the only report available which is based on anti-aggregation of Au NPs.

Figure 11. Schematic representation of the colorimetric assay of Pi based on anti-aggregation of MA-Au NPs (reproduced with the permission from ^[74]).

The CI-Au NPs (particle size = 13 nm) were also engaged in anti-aggregation-enabled detection of the nitrite anion (NO₂⁻) using 4-Amino-thiophenol (4-ATP; 25 µM) as the aggregation-inducing agent. Herein, the 4-ATP induced aggregation (with a color change from red to blue) via a coating on Au NPs’ surface; however, the diazonium reaction with NO₂⁻ disturbed the aggregation and led to a blue-to-red color change. To attain the best result, the pH, H₂SO₄ concentration, and incubation time were fixed as pH 9.5 (buffer: Na₂CO₃–NaHCO₃), 100 mM H₂SO₄ (catalytic amount of 10 µL from 1M stock) and 15 min, respectively. SPR, TEM, and interference studies validated the proposed tactic for NO₂⁻ quantitation. The linear range of NO₂⁻ was between 1 and 25 µM (at A₆₈₅/A₅₂₀), with an LOD of 1 µM. Spiked ground water and tap water investigations on the NO₂⁻ quantification showed recoveries of >98%, with an RSD of ≤8%, thereby confirming the practicality of this method. Based on the available data, this report can be regarded as an exceptional work for the NO₂⁻ assay.

4. Ag NPs-Based Colorimetric Assay of Diverse Analytes via Anti-Aggregation

Silver nanoparticles (Ag NPs) were also reported toward distinct analyte quantification via the anti-aggregation strategy similar to Au NPs, as discussed in this section. Duan et al. described the use of citrate-capped Ag NPs (CI-Ag NPs) toward the detection of Hg²⁺ via anti-aggregation in the presence of the aggregation-inducing agent 6-thioguanine (0.83 µM) ^[75], in which 6-thioguanine induced aggregation of Ag NPs to produce a color change from yellow to brown. However, the brown color was reversed to yellow via anti-aggregation by pre-mixing Hg²⁺ (at different concentrations). The underlying mechanism was that 6-thioguanine induced aggregation due to its strong electrostatic interaction toward CI-Ag NPs. However, the aggregation was inhibited by the presence of Hg²⁺ (via strong binding of 6-thioguanine with Hg²⁺) and resulted in the anti-aggregation of CI-Ag NPs. The best colorimetric response was achieved at pH 4 (100 mM HAc-NaAc) after 30 min incubation. The linear range of Hg²⁺ was between 0 and 333 nM (at A₅₃₀/A₃₉₄), with an LOD of 4 nM. SPR, TEM, interference, and spring water studies (recoveries of between 103 and 105%) validated this work as a major innovation. Figure 12A–C display the colorimetric response, SPR variations, and linearity of Hg²⁺ induced anti-aggregation of 6-thioguanine-CI-Ag NPs system, respectively.

Figure 12. (A) The photographic images of the 6-thioguanine–Ag NPs solutions in the presence of different concentrations of Hg²⁺ (0–600 nM). (B) The corresponding UV–vis spectra of 6-thioguanine–Ag NPs. (C) The absorbance ratio (A₅₃₀/A₃₉₄) of 6-thioguanine–Ag NPs versus Hg²⁺ concentration. Inset to C: plot of A₅₃₀/A₃₉₄ versus the Hg²⁺ concentration. The concentration of 6-thioguanine was 0.83 μM ((A–C) are reproduced with the permission from ^[75]).

Anti-aggregation-enabled colorimetric assay of Cu²⁺ was demonstrated by employing CI-Ag NPs and engaging 1,4-dithiothreitol (DTT; 10 µM) as the aggregation-inducing agent ^[76], wherein DTT induced the aggregation of Ag NPs via a strong Ag-S bonding and displayed a color change from yellow to deep green. Upon pre-mixing Cu²⁺ in 2,6-pyridinedicarboxylic acid (PDCA, 5 mM), the DTT oxidized to form an intramolecular-S-S-bond, which inhibited aggregation and produced a reversed color change from deep green to yellow. The best result was attained at pH 2–10 (buffer: HCl-NaOH) after 20 min incubation. The linearity of Cu²⁺ was between 0.1 and 2 µM (at A₄₀₈), with an LOD of 0.1 µM. SPR, TEM, interference, and real samples (tap water and ground water; recoveries of ≥95%, with an RSD of <8%) interrogations validated this method. Hence, it can be regarded as a remarkable research, but details on particle size variations are still missing.

He and co-workers reported the anti-aggregation-enabled colorimetric detection of Mn²⁺ by employing CI-Ag NPs (conc. = 0.28 nM) and L-Arginine (L-Arg; 0.7 mM) as the aggregation-inducing agent ^[77]. The Ag-N covalent interaction between Ag NPs and L-Arg caused aggregation, with the color of the solution changing from yellow to colorless. The color of Ag NPs changed back from colorless to yellow via anti-aggregation when the above solution was pre-treated with different concentrations of Mn²⁺. In fact, the anti-aggregation was attributed to the stronger attraction of L-Arg toward Mn²⁺ than that of the Ag NPs. The best result was obtained at pH 9.4 (buffer; BR) after 40 min incubation. Two linear ranges for Mn²⁺ were established between 0 and 700 nM and 5 and 70 µM (both at A₃₉₀), with an LOD of 20 nM. As illustrated in Figure 13A,B, none of the competing species shows interference to Mn²⁺ selectivity. By this method, real water (lake, tap, and river) recoveries were >90%, with an RSD < 3%. This work is regarded as an exceptional innovation, but it still lacks particle size variations details.

Figure 13. (A) The photographs and (B) the corresponding absorbance of the Ag NPs-l-arginine dispersions in the presence of different metal ions. The concentrations of both Mn²⁺ ions and other metal ions were 10 µM ((A,B) are reproduced with the permission from ^[77]).

Basiri et al. developed the Ag NPs–decorated graphene nanocomposites (rGO@Ag NPs; particle size = 3.7 ± 0.8 nm) for anti-aggregation-enabled colorimetric detection of Cu²⁺ using dopamine (DA; 25 µM) as the aggregation-inducing agent. The Ag NPs and 5 mg/mL of rGO was boiled at pH 10 and at 100 °C for 10 min to afford rGO@Ag NPs. The rGO@Ag NPs were well characterized by SPR, TEM, FTIR, XPS, and XRD studies (XRD = X-ray diffraction). Due to the H-bonding and π–π interactions between DA and rGO@Ag NPs, aggregation occurred (witnessed by TEM) with a color change from yellow to brown. In the presence of Cu²⁺, the interaction of DA to rGO@Ag NPs was hindered due to selective chelation of Cu²⁺ with DA and resulted in anti-aggregation (seen by TEM) with a color change from brown to yellow. Note that none of the competing species induces anti-aggregation, as shown in Figure 14. The best colorimetric response was observed at pH 7 (buffer: 10 mM PBS) after 15 min incubation. The linearity of Cu²⁺ ranged between 0.02 and 1.5 µM (at A₄₀₅/A₅₁₅), with an LOD of 9.8 nM. Moreover, real samples (human urine for DA and tomato for Cu²⁺) investigations demonstrated recoveries of >98%, with an RSD of <3%; thus, it can be noted as an excellent method.

Figure 14. Colorimetric signals in the presence of different metal ions and anions (1.5 µM) as the interferences of Cu²⁺. Inset: the photographic images of the corresponding solutions (reproduced with the permission from ^[78]).

An anti-aggregation-enabled colorimetric assay of Br⁻ and I⁻ was demonstrated using CI-Ag NPs (particle size = 10–15 nm; conc. = 69.4 µM) and Cr³⁺ (125 µM) as the aggregation-inducing agent ^[79]. In the presence of Cr³⁺, the Ag NPs aggregated to produce a color change from yellow to brown. Due to the complexation of Cr³⁺ with Br⁻ and I⁻, anti-aggregation occurred to reverse the color from brown to yellow. The linearity of Br^– and I^– was between 0.99 and 5.66 µM (at A₅₀₀) and 0.99 and 4.16 µM (at A₅₀₀), with LODs of 1.67 µM and 1.32 µM, respectively. Though this report was validated by SPR, DLS, and interference studies, data in TEM, pH, incubation time, and real-time investigations were insufficient; therefore, it can be only regarded as a preliminary work. To this track, the sulfanilic acid (SA) and catechol (CAT)–functionalized Ag NPs aggregate (SA-CAT-Ag NPs) was utilized for anti-aggregation-enabled colorimetric detection of F^– ^[80]. The CA was firstly oxidized by silver ions followed by SA nucleophilic attack to deliver aggregated SA-CAT-Ag NPs with red color. By adding F^– to the above mixture, it adsorbed on the Ag surface and stripped the SA-CAT stabilizer from -SO₃⁻ moiety, hence the aggregation was disturbed with a color change from red to yellow. The best color response was attained at pH 4–9 after 1 min incubation. The linear range of F^– was between 1 and 40 µM (at A₃₉₇/A₅₀₈), with an LOD of 0.2 µM. This work was authenticated by SPR, TEM, interference (negligible), and spring water investigations. Thus, it can be noted as a nice approach for F^– quantification.

Dong et al. demonstrated using poly-vinyl pyridine (PVP)–modified Ag NPs (PVP-Ag NPs; particle size = 6.24 nm) the colorimetric assay pyrophosphate ((P₂O₇⁴⁻; PPi) via the anti-aggregation strategy ^[81]. Herein, the Pb²⁺ (5 µM) was employed as the aggregation-inducing agent to induce the PVP-Ag NPs (due to its crosslink effect with PVP) aggregation and to produce a color change from yellow to blue. Anti-aggregation was induced by pre-mixing PPi, which resulted in a colorimetric response from blue to yellow due to the chelating interaction between PPi and Pb²⁺. The best result was obtained at pH 5–9 at 20 °C after 30 min incubation. The linear ranges of PPi were between 0.2 and 2.0 μM and 2.0 and 10.0 μM (at A₃₉₆), with an LOD of 0.2 µM. This work was authorized by smartphone device, SPR, TEM, statistical, theoretical, interference, real samples (tap water (recoveries between 79.6 and 107.6%) and canned meat (66.7–118.4%)) interrogations. Therefore, it can be regarded as an exceptional invention in Ag NPs-based colorimetric assay. Following a similar mechanistic approach, the 2-Mercapto-ethane sulfonate–modified silver nanoplates (MS-Ag NPls) were employed in the colorimetric detection of PO₄³⁻ (Pi) on PADs in the presence of the aggregation-inducing agent Eu³⁺ (65.81 µM) ^[82]. In the presence of Eu³⁺, the color of MS-Ag NPls changed from pink to purple, which was reversed (from purple to pink) during the quantification of Pi. All the sensory experiments were conducted at pH 7 (buffer: 25 mM Tris) at an optimum incubation time of 3 min. The linear regression of Pi was between 10.42 and 313 µM (at A₅₂₀), with an LOD of 3.44 µM. This work was also authenticated by water and soil investigations. Moreover, this is the only report available on the Ag NPls-based colorimetric assay. Therefore, it is regarded as a remarkable method.

The surface-enhanced Raman scattering (SERS) technique was employed with 4-mercaptopyridine (4-MPY)-modified Ag NPs (4-MPY-Ag NPs) for anti-aggregation-enabled detection of trypsin (an enzyme that aids with digestion) in the presence of the aggregation-inducing agent protamine (0.3 µg/mL (10 µL)) ^[83]. The presence of +vely charged protamine over 4-MPY-Ag NPs induced aggregation via strong electrostatic interaction with a color change from yellow to deep green. However, trypsin enhanced the enzymatic fragmentation of protamine, which resulted in anti-aggregation with a reversed color change from deep green to yellow. The SERS linear range was between 0.14 nM and 13.8 μM (at 1096 cm⁻¹), with an LOD of 0.14 nM. All the SERS experiments were conducted at pH 7.4 (buffer: HEPES) within 1 min. Based on SERS, TEM, and interference studies, this work can be regarded as a unique one. An anti-aggregation-enabled colorimetric assay of poly diallyl-dimemethyl-ammonium chloride (PDADMAC; a coagulant in water and wastewater treatment) was proposed by employing CI-Ag NPs (particle size = 6 ± 2 nm) and 0.2 M PBS buffer (pH 7.4) as the aggregation-inducing agent ^[84]. The phosphate buffer induced aggregation of Ag NPs, which resulted in a color change from yellow to colorless. However, in the presence of PDADMAC, the Ag NPs were stabilized by electrostatic attraction and steric hindrance, which resulted in being dispersed NPs with a reversed color change (from colorless to yellow). Note that the colorimetric responses occurred within 3 min. The linear range of PDADMAC was between 1 and 100 mg. L⁻¹ (at A₃₉₆), with an LOD of 0.7 mg. L⁻¹. This tactic was well authorized by SPR, TEM, interference, and tap water interrogations. Thus, it can be noted as a nice innovation. However, further optimization is mandatory toward the LOD reduction.

Though anti-aggregation-enabled Ag NPs-based colorimetric assays seem to be effective in analytes quantification, available reports are much less sufficient than that of aggregated Ag NPs-based colorimetric sensors ^[85]^[86]^[87]^[88]^[89]^[90]. Therefore, further research is required as suggested in perspectives.

5. Design/Sensory Requirements, Advantages, and Limitations

5.1. Probe Design and Sensory Requirements

The development of exceptional Au NPs- and Ag NPs-based colorimetric probes and their anti-aggregation-enabled sensory performances must follow the requirements as stated below:

The selected label-free/modified Au NPs- or Ag NPs-based probes must possess dispersion stability in color and firm electrostatic charges to be able to interact with aggregation-inducing agents and to afford a certain degree of aggregation and a distinct colorimetric response.
Selection of a suitable aggregation-inducing agent is essential to attain the primary colorimetric changes. An aggregation-inducing agent should have any of the following three characteristics: (i) analyte replaceable electrostatic interactions, (ii) analyte disposable interactive binding (such as Au-S, Ag-S, Au-N, Ag-N, etc.) over the NPs’ surface, and (iii) analyte coordinative units (such as thymine, -SH, -NH₂, -COOH, -OCH₃, and -OC₂H₅, etc.) that are able to form coordination complexes or undergo catalytic reactions to be released from the NPs’ surface. Moreover, it is essential to fix the concentration of the aggregation-inducing agent before taking further sensory interrogations.
Optimizations of the effective pH conditions, suitable pH buffer solution, concentration, and maintenance circumstances are necessary for anti-aggregation-based colorimetric analyte detection.
It is essential to fix the exact incubation time to achieve stable colorimetric response and to help to clarify the underlying mechanism and kinetics of the anti-aggregation.
Though most of the anti-aggregation-enabled colorimetric sensory are conducted at room temperature, it is also important to fix the operative temperature toward real-time applications.
To avoid complications in real water investigations, it is necessary to identify the interfering effect, allowed concentration, required masking agent, etc.

5.2. Advantages

Anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric assays have following advantages, as stated below:

In general, anti-aggregation-enabled Au NPs- and Ag NPs-based analyte quantifications are conducted in an aqueous environment; therefore, they are tactics comparable to the available luminescent inorganic and organic nanomaterials-based sensors performed in diverse solvent conditions [122–130].
The anti-aggregation-enabled Au NPs- and Ag NPs-based analyte detection can be observed with naked eye, thereby becoming a competing strategy to the reported organic dyes-based sensors ^[91]^[92]^[93]^[94]^[95]^[96]^[97].
In the anti-aggregation-enabled sensory studies, sequential colorimetric detection of the aggregation-inducing agent and analyte can be carried out with reliable linearity and LODs down to nano- and pico-molar levels. Thus, it can be regarded as an advantage over other methods.
By using the anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric assay, diverse analytes can be detected with specificity under a fixed pH value, incubation time, and operative temperature.
Anti-aggregation of Au NPs in the presence of certain peptide sequences can be employed to avoid amyloid aggregation and Alzheimer’s disease occurrence.
The anti-aggregation strategy in Au NPs- and Ag NPs-based colorimetric sensors can be effectively applied in real samples, such as water, soil, plasma, urine, etc.; thus, it can be noted as a great advantage toward development of the “state of the art” method.

5.3. Limitations

Anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensors also have a few limitations, as discussed below:

Anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensory responses are aquatic environment-dependent; therefore, the detection of analytes in other solvent systems is not possible with this method.
There are many optimization conditions (such as the aggregation-inducing agent, pH (buffer), NaCl effect, incubation time, temperature, etc.) required in order to attain the sensory responses, thereby limiting the frequent use of the anti-aggregation strategy.
Due to the co-existence of the aggregation-inducing agent and analyte in the same mixture, the re-use of the mixture is not possible at all. Similarly, many reports used complicated assay protocols, which may have led to unknown human errors and restricted the use of this method.
Elucidation of precise sensory mechanisms requires supportive evidence, such as FTIR, TEM, XPS, XRD, etc. Thus, this method was limited by available instruments and cost-effectiveness.
The best result of anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensors can only be attained at a fixed interfering specie concentration, which is not possible with real samples (may contain large amounts of interference); therefore, real-time use of the strategy is still questionable.
Real water/soil samples from different places contain various complicated matrixes. Thus, applying the anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric assay to all the real samples is not possible or may not deliver expected results.

6. Conclusions and Perspectives

In this entry, discussions on anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensing of metal ions, anions, bio-analytes, pesticides, and herbicides are delivered in great details. The assay protocols, optimized concentrations of aggregation-inducing agents, pH (buffer solution), incubation times, and operating temperatures are provided and tabulated for readers. The exact underlying mechanisms of each probe leading to the colorimetric response are explained with clear evidence. Real-time applications are illustrated, and comments are given on the performance of each individual report. Finally, researchers suggest the probe design/sensory requirements together with the merits and limitations of the anti-aggregation strategy. Though the anti-aggregation strategy-based colorimetric sensors are noted as being great innovations, there are still a few perspective points that need to be focused on, as noted below:

Most reports on anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensors used complicated assay protocols, which should be simplified toward “state of the art”.
Performance of anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensors depends on the pH values and buffer used. The use of unified pH buffer solution must be justified in the future to achieve a unique anti-aggregation strategy.
By means of anti-aggregation, numerous reports are available on Au NPs-based colorimetric assay of Hg²⁺; however, none of them were commercialized toward practicality. This requires more efforts in future.
The majority of anti-aggregation-based colorimetric sensors by Au NPs and Ag NPs focused on single-analyte quantification so far, which must be upgraded to multi-analyte detection. For example, the aggregation-inducing agent Hg²⁺ also has the coordination tendency for biothiols and I⁻, hence it can be employed in the quantification of both analytes.
In many reports, anti-aggregation due to coordination of aggregation agents with analytes was not supported by valid evidence, which requires further clarification.
Currently, anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensors are only available for the detection of limited metal analytes (such as Hg²⁺, Cu²⁺,Ag⁺, Mn²⁺, and Sc³⁺), thereby requiring more attention toward developing discrimination of more metal ion species.
Anti-aggregation proposed for oxidative catalytic reactions falls short of evidence, and it should be addressed with more experimental data.
Concerning the social impact, anti-aggregation-enabled in vitro colorimetric assay of amyloid β peptide toward the inhibition of Alzheimer disease requires great attention in future research work.
Reports on anti-aggregation-enabled Au-NPs colorimetric assay of anions, pesticides, and herbicides are insufficient to justify the effectiveness of the strategy toward those analytes. This issue should be addressed in future.
Only a few reports are available on the anti-aggregation-stimulated Ag NPs-based colorimetric sensors toward diverse analytes detection. Therefore, research on anti-aggregation of Ag NPs toward the discrimination of distinct analytes requires great attention.
Compositions of Au NPs and Ag NPs with other established luminescent nanomaterials can be the focus to afford anti-aggregation-based colorimetric and luminescent responses toward specific analytes.
Many anti-aggregation-based sensory reports lack theoretical supports, which should be addressed using density functional theory (DFT) investigations in future.

Though there are still many unclearly issues requiring further clarification, colorimetric recognition of specified analytes via anti-aggregation is becoming an important research field. Currently, there are many groups working on the development of new anti-aggregation-enabled Au NPs- and Ag NPs-based colorimetric sensory probes to remedy the aforementioned issues. However, the anti-aggregation strategy can be regarded as exceptional for analyte quantification in terms of naked eye detection ability and real-time applicability.

References

Mitra, S.; Chakraborty, A.J.; Tareq, A.M.; Emran, T.B.; Nainu, F.; Khusro, A.; Idris, A.M.; Khandaker, M.U.; Osman, H.; Alhumaydhi, F.A.; et al. Impact of Heavy Metals on the Environment and Human Health: Novel Therapeutic Insights to Counter the Toxicity. J. King Saud Univ. Sci. 2022, 34, 101865.
Gonçalves, A.R.P.; Paredes, X.; Cristino, A.F.; Santos, F.J.V.; Queirós, C.S.G.P. Ionic Liquids—A Review of Their Toxicity to Living Organisms. Int. J. Mol. Sci. 2021, 22, 5612.
Hua, Y.; Ma, J.; Li, D.; Wang, R. DNA-Based Biosensors for the Biochemical Analysis: A Review. Biosensors 2022, 12, 183.
Aktar, W.M.; Sengupta, D.; Chowdhury, A. Impact of Pesticides Use in Agriculture: Their Benefits and Hazards. Interdisc Toxicol. 2009, 2, 1–12.
Gianessi, L.P. The Increasing Importance of Herbicides in Worldwide Crop Production. Pest Manag. Sci. 2013, 69, 1099–1105.
Alberti, G.; Zanoni, C.; Magnaghi, L.R.; Biesuz, R. Gold and Silver Nanoparticle-Based Colorimetric Sensors: New Trends and Applications. Chemosensors 2021, 9, 305.
Prosposito, P.; Burratti, L.; Venditti, I. Silver Nanoparticles as Colorimetric Sensors for Water Pollutants. Chemosensors 2020, 8, 26.
Walekar, L.; Dutta, T.; Kumar, P.; Ok, Y.S.; Pawar, S.; Deep, A.; Kim, K.-H. Functionalized Fluorescent Nanomaterials for Sensing Pollutants in the Environment: A Critical Review. TrAC Trends Anal. Chem. 2017, 97, 458–467.
Bulska, E.; Ruszczyńska, A. Analytical Techniques for Trace Element Determination. Phys. Sci. Rev. 2017, 2, 20178002.
Vervoort, N.; Goossens, K.; Baeten, M.; Chen, Q. Recent Advances in Analytical Techniques for High Throughput Experimentation. Anal. Sci. Adv. 2021, 2, 109–127.
Kreno, L.E.; Leong, K.; Farha, O.K.; Allendorf, M.; Van Duyne, R.P.; Hupp, J.T. Metal–Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112, 1105–1125.
Shellaiah, M.; Sun, K.-W. Progress in Metal-Organic Frameworks Facilitated Mercury Detection and Removal. Chemosensors 2021, 9, 101.
Auwalu, M.A.; Cheng, S. Diketopyrrolopyrrole Fluorescent Probes, Photophysical and Biological Applications. Chemosensors 2021, 9, 44.
Shellaiah, M.; Chen, Y.-T.; Thirumalaivasan, N.; Aazaad, B.; Awasthi, K.; Sun, K.W.; Wu, S.-P.; Lin, M.-C.; Ohta, N. Pyrene-Based AIEE Active Nanoprobe for Zn2+ and Tyrosine Detection Demonstrated by DFT, Bioimaging, and Organic Thin-Film Transistor. ACS Appl. Mater. Interfaces 2021, 13, 28610–28626.
Cichosz, S.; Masek, A.; Zaborski, M. Polymer-Based Sensors: A Review. Poly. Test. 2018, 67, 342–348.
Shellaiah, M.; Simon, T.; Thirumalaivasan, N.; Sun, K.W.; Ko, F.-H.; Wu, S.-P. Cysteamine-Capped Gold-Copper Nanoclusters for Fluorometric Determination and Imaging of Chromium(VI) and Dopamine. Microchim. Acta 2019, 186, 788.
Terai, T.; Nagano, T. Fluorescent Probes for Bioimaging Applications. Curr. Opin. Chem. Biol. 2008, 12, 515–521.
Sahoo, S.K.; Crisponi, G. Recent Advances on Iron(III) Selective Fluorescent Probes with Possible Applications in Bioimaging. Molecules 2019, 24, 3267.
Shellaiah, M.; Awasthi, K.; Chandran, S.; Aazaad, B.; Sun, K.W.; Ohta, N.; Wu, S.-P.; Lin, M.-C. Methylammonium Tin Tribromide Quantum Dots for Heavy Metal Ion Detection and Cellular Imaging. ACS Appl. Nano Mater. 2022, 5, 2859–2874.
Shellaiah, M.; Sun, K.-W. Pyrene-Based AIE Active Materials for Bioimaging and Theranostics Applications. Biosensors 2022, 12, 550.
Chang, C.-C.; Chen, C.-P.; Wu, T.-H.; Yang, C.-H.; Lin, C.-W.; Chen, C.-Y. Gold Nanoparticle-Based Colorimetric Strategies for Chemical and Biological Sensing Applications. Nanomaterials 2019, 9, 861.
Hussain, M.; Nafady, A.; Sirajuddin; Avcı, A.; Pehlivan, E.; Nisar, J.; Sherazi, S.T.H.; Balouch, A.; Shah, M.R.; Almaghrabi, O.A.; et al. Biogenic Silver Nanoparticles for Trace Colorimetric Sensing of Enzyme Disrupter Fungicide Vinclozolin. Nanomaterials 2019, 9, 1604.
Shellaiah, M.; Simon, T.; Sun, K.W.; Ko, F.-H. Simple Bare Gold Nanoparticles for Rapid Colorimetric Detection of Cr3+ Ions in Aqueous Medium with Real Sample Applications. Sens. Actuators B 2016, 226, 44–51.
Zhao, L.; Zhao, L.; Miao, Y.; Liu, C.; Zhang, C. A Colorimetric Sensor for the Highly Selective Detection of Sulfide and 1,4-Dithiothreitol Based on the In-Situ Formation of Silver Nanoparticles Using Dopamine. Sensors 2017, 17, 626.
Dawadi, S.; Katuwal, S.; Gupta, A.; Lamichhane, U.; Thapa, R.; Jaisi, S.; Lamichhane, G.; Bhattarai, D.P.; Parajuli, N. Current Research on Silver Nanoparticles: Synthesis, Characterization, and Applications. J. Nanomater. 2021, 2021, 6687290.
Shellaiah, M.; Simon, T.; Venkatesan, P.; Sun, K.W.; Ko, F.-H.; Wu, S.-P. Nanodiamonds Conjugated to Gold Nanoparticles for Colorimetric Detection of Clenbuterol and Chromium(III) in Urine. Microchim. Acta 2017, 185, 74.
Sanskriti, I.; Upadhyay, K.K. Twinning as a Guiding Factor in Morphological Anisotropy of Silver Nanoparticles Stabilized Over L–DOPA: A Colorimetric Probe for Sulfide in Aqueous Medium. ChemistrySelect 2019, 4, 3803–3810.
Shellaiah, M.; Thirumalaivasan, N.; Sun, K.W.; Wu, S.-P. A pH Cooperative Strategy for Enhanced Colorimetric Sensing of Cr(III) Ions Using Biocompatible L-Glutamic Acid Stabilized Gold Nanoparticles. Microchem. J. 2021, 160, 105754.
Singh, R.; Mehra, R.; Walia, A.; Gupta, S.; Chawla, P.; Kumar, H.; Thakur, A.; Kaushik, R.; Kumar, N. Colorimetric Sensing Approaches Based on Silver Nanoparticles Aggregation for Determination of Toxic Metal Ions in Water Sample: A Review. Inter. J. Environ. Anal. Chem. 2021, 1–16.
Sabela, M.; Balme, S.; Bechelany, M.; Janot, J.-M.; Bisetty, K. A Review of Gold and Silver Nanoparticle-Based Colorimetric Sensing Assays. Adv. Eng. Mater. 2017, 19, 1700270.
Guo, L.; Xu, Y.; Ferhan, A.R.; Chen, G.; Kim, D.-H. Oriented Gold Nanoparticle Aggregation for Colorimetric Sensors with Surprisingly High Analytical Figures of Merit. J. Am. Chem. Soc. 2013, 135, 12338–12345.
Zhang, L.; Yuan, Y.; Wen, X.; Li, Y.; Cao, C.; Xiong, Q. A Coordination and Ligand Replacement Based Three-Input Colorimetric Logic Gate Sensing Platform for Melamine, Mercury Ions, and Cysteine. RSC Adv. 2015, 5, 59106–59113.
Wang, N.; Yang, H.-F.; Zhu, X.; Zhang, R.; Wang, Y.; Huang, G.-F.; Zhang, Z.-R. Synthesis of Anti-aggregation Silver Nanoparticles Based on Inositol Hexakisphosphoric Micelles for A Stable Surface Enhanced Raman Scattering Substrate. Nanotechnology 2009, 20, 315603.
Yang, L.; Wang, N.; Zheng, G. Enhanced Effect of Combining Chlorogenic Acid on Selenium Nanoparticles in Inhibiting Amyloid β Aggregation and Reactive Oxygen Species Formation In Vitro. Nanoscale Res. Lett. 2018, 13, 303.
Yang, L.; Wang, W.; Chen, J.; Wang, N.; Zheng, G. A Comparative Study of Resveratrol and Resveratrol-Functional Selenium Nanoparticles: Inhibiting Amyloid β Aggregation and Reactive Oxygen Species Formation Properties. J. Biomed. Mater. Res. A 2018, 106, 3034–3041.
Li, M.; Shi, P.; Xu, C.; Ren, J.; Qu, X. Cerium Oxide Caged Metal Chelator: Anti-aggregation and Anti-oxidation Integrated H2O2-responsive Controlled Drug Release for Potential Alzheimer’s Disease Treatment. Chem. Sci. 2013, 4, 2536–2542.
Ishtikhar, M.; Rahisuddin; Khan, M.V.; Khan, R.H. Anti-aggregation Property of Thymoquinone Induced by Copper-Nanoparticles: A Biophysical Approach. Inter. J. Biol. Macromol. 2016, 93, 1174–1182.
Kong, L.; Zhou, X.; Shi, G.; Yu, Y. Molybdenum Disulfide Nanosheets-Based Fluorescent “Off-to-On” Probe for Targeted Monitoring and Inhibition of β-Amyloid Oligomers. Analyst 2020, 145, 6369–6377.
Duangdeewong, C.; Choengchan, N.; Wattanasin, P.; Teerasong, S. Direct Determination of Ethanol in Alcoholic Beverages Based on Its Anti-aggregation of Melamine-Silver Nanoparticle Assembly. Talanta 2022, 250, 123751.
Balali-Mood, M.; Naseri, K.; Tahergorabi, Z.; Khazdair, M.R.; Sadeghi, M. Toxic Mechanisms of Five Heavy Metals: Mercury, Lead, Chromium Cadmium, and Arsenic. Front. Pharmacol. 2021, 12, 643972.
Lechuga, M.; Fernández-Serrano, M.; Jurado, E.; Núñez-Olea, J.; Ríos, F. Acute Toxicity of Anionic and Non-ionic Surfactants to Aquatic Organisms. Ecotoxicol. Environ. Safety 2016, 125, 1–8.
Wang, T.; Ramnarayanan, A.; Cheng, H. Real Time Analysis of Bioanalytes in Healthcare, Food, Zoology and Botany. Sensors 2018, 18, 5.
Dai, J.; Ma, C.; Zhang, P.; Fu, Y.; Shen, B. Recent Progress in the Development of Fuorescent Probes for Detection of Biothiols. Dyes Pig. 2020, 177, 108321.
Defarge, N.; Spiroux de Vendômois, J.; Séralini, G.E. Toxicity of Formulants and Heavy Metals in Glyphosate-Based Herbicides and Other Pesticides. Toxicol. Rep. 2018, 5, 156–163.
DeLorenzo, M.E.; Scott, G.I.; Ross, P.E. Toxicity of Pesticides to Aquatic Microorganisms: A Review. Environ. Toxicol. Chem. 2001, 20, 84–98.
Najafzadeh, F.; Ghasemi, F.; Hormozi-Nezhad, M.R. Anti-aggregation of Gold Nanoparticles for Metal Ion Discrimination: A Promising Strategy to Design Colorimetric Sensor Arrays. Sens. Actuators B 2018, 270, 545–551.
Li, Y.; Wu, P.; Xu, H.; Zhang, Z.; Zhong, X. Highly Selective and Sensitive Visualizable Detection of Hg2+ Based on Anti-aggregation of Gold Nanoparticles. Talanta 2011, 84, 508–512.
Ding, N.; Zhao, H.; Peng, W.; He, Y.; Zhou, Y.; Yuan, L.; Zhang, Y. A Simple Colorimetric Sensor Based on Anti-aggregation of Gold Nanoparticles for Hg2+ Detection. Colloids Surf. A 2012, 395, 161–167.
Lou, T.; Chen, L.; Zhang, C.; Kang, Q.; You, H.; Shen, D.; Chen, L. A Simple and Sensitive Colorimetric Method for Detection of Mercury Ions Based on Anti-aggregation of Gold Nanoparticles. Anal. Methods 2012, 4, 488–491.
Li, Y.-L.; Leng, Y.-M.; Zhang, Y.-J.; Li, T.-H.; Shen, Z.-Y.; Wu, A.-G. A New Simple and Reliable Hg2+ Detection System Based on Anti-aggregation of Unmodified Gold Nanoparticles in the Presence of O-phenylenediamine. Sens. Actuators B 2014, 200, 140–146.
Huang, G.G.; Chen, Y.-T.; Lin, Y.-R. Development of A Gold Nanoparticle Based Anti-aggregation Method for Rapid Detection of Mercury(ii) in Aqueous Solutions. Anal. Methods 2014, 6, 5690–5696.
Zhou, Y.; Dong, H.; Liu, L.; Li, M.; Xiao, K.; Xu, M. Selective and Sensitive Colorimetric Sensor of Mercury (II) Based on Gold Nanoparticles and 4-Mercaptophenylboronic Acid. Sens. Actuators B 2014, 196, 106–111.
Tang, J.; Wu, P.; Hou, X.; Xu, K. Modification-free and N-Acetyl-L-cysteine-Induced Colorimetric Response of AuNPs: A Mechanistic Study and Sensitive Hg2+ Detection. Talanta 2016, 159, 87–92.
Jin, W.; Huang, P.; Wei, G.; Cao, Y.; Wu, F. Visualization and Quantification of Hg2+ Based on Anti-aggregation of Label-free Gold Nanoparticles in the Presence of 2-Mercaptobenzothiazole. Sens. Actuators B 2016, 233, 223–229.
Rajeshwari, A.; Karthiga, D.; Chandrasekaran, N.; Mukherjee, A. Anti-aggregation-Based Spectrometric Detection of Hg(II) at Physiological pH Using Gold Nanorods. Mater. Sci. Eng. C 2016, 67, 711–716.
Sun, X.; Liu, R.; Liu, Q.; Fei, Q.; Feng, G.; Shan, H.; Huan, Y. Colorimetric Sensing of Mercury (II) Ion Based on Anti-aggregation of Gold Nanoparticles in the Presence of Hexadecyl Trimethyl Ammonium Bromide. Sens. Actuators B 2018, 260, 998–1003.
Kataria, R.; Sethuraman, K.; Vashisht, D.; Vashisht, A.; Mehta, S.K.; Gupta, A. Colorimetric Detection of Mercury Ions Based on Anti-aggregation of Gold Nanoparticles Using 3, 5-Dimethyl-1-thiocarboxamidepyrazole. Microchem. J. 2019, 148, 299–305.
Mao, L.; Wang, Q.; Luo, Y.; Gao, Y. Detection of Ag+ Ions Via an Anti-aggregation Mechanism Using Unmodified Gold nanoparticles in the Presence of Thiamazole. Talanta 2021, 222, 121506.
Safavi, A.; Ahmadi, R.; Mohammadpour, Z. Colorimetric Sensing of Silver ion Based on Anti Aggregation of Gold Nanoparticles. Sens. Actuators B 2017, 242, 609–615.
Selva Sharma, A.; SasiKumar, T.; Ilanchelian, M. A Rapid and Sensitive Colorimetric Sensor for Detection of Silver Ions Based on the Non-aggregation of Gold Nanoparticles in the Presence of Ascorbic Acid. J. Cluster Sci. 2018, 29, 655–662.
Gao, Q.; Zheng, Y.; Song, C.; Lu, L.-Q.; Tian, X.-K.; Xu, A.-W. Selective and Sensitive Colorimetric Detection of Copper Ions Based on Anti-aggregation of the Glutathione-Induced Aggregated Gold Nanoparticles and Its Application for Determining Sulfide Anions. RSC Adv. 2013, 3, 21424–21430.
Hormozi-Nezhad, M.R.; Abbasi-Moayed, S. A Sensitive and Selective Colorimetric Method for Detection of Copper Ions Based on Anti-aggregation of Unmodified Gold Nanoparticles. Talanta 2014, 129, 227–232.
Deng, H.-H.; Huang, K.-Y.; Fang, Q.-H.; Lv, Y.-P.; He, S.-B.; Peng, H.-P.; Xia, X.-H.; Chen, W. Schiff Base and Lewis Acid-Base Interaction-Regulated Aggregation/Dispersion of Gold Nanoparticles for Colorimetric Recognition of Rare-Earth Sc3+ Ions. Sens. Actuators B 2020, 311, 127925.
Plaisen, S.; Cheewasedtham, W.; Rujiralai, T. Robust Colorimetric Detection Based on the Anti-aggregation of Gold Nanoparticles for Bromide in Rice Samples. RSC Adv. 2018, 8, 21566–21576.
Chen, L.; Lu, W.; Wang, X.; Chen, L. A Highly Selective and Sensitive Colorimetric Sensor for Iodide Detection Based on Anti-aggregation of Gold Nanoparticles. Sens. Actuators B 2013, 182, 482–488.
Zhou, G.; Zhao, C.; Pan, C.; Li, F. Highly Sensitive and Selective Colorimetric Detection of Iodide Based on Anti-Aggregation of Gold Nanoparticles. Anal. Methods 2013, 5, 2188–2192.
Rasouli, Z.; Ghavami, R. Colorimetric Sensing of Iodide by the Competitive Interactions in the Surface of Gold Nanoparticles with the Simultaneous Aggregation/ Anti-Aggregation Mechanisms in Edible Salts. Current Anal. Chem. 2018, 14, 539–547.
Peng, R.; He, H.; Wang, Q.; Yan, X.; Yu, Q.; Qin, H.; Lei, Y.; Luo, L.; Feng, Y. Cu(Ⅱ) Triggering Redox-regulated Anti-aggregation of Gold Nanoparticles for Ultrasensitive Visual Sensing of Iodide. Anal. Chim. Acta 2018, 1036, 147–152.
Pournaghi, A.; Keshvari, F.; Bahram, M. Colorimetric Determination of Iodine Based on Highly Selective and Sensitive Anti-aggregation Assay. J. Iran. Chem. Soc. 2019, 16, 143–149.
Deng, H.-H.; Wu, C.-L.; Liu, A.-L.; Li, G.-W.; Chen, W.; Lin, X.-H. Colorimetric Sensor for Thiocyanate Based on Anti-aggregation of Citrate-Capped Gold Nanoparticles. Sens. Actuators B 2014, 191, 479–484.
Song, J.; Huang, P.-C.; Wan, Y.-Q.; Wu, F.-Y. Colorimetric Detection of Thiocyanate Based on Anti-aggregation of Gold Nanoparticles in the Presence of Cetyltrimethyl Ammonium Bromide. Sens. Actuators B 2016, 222, 790–796.
Zhao, Y.; Liu, R.; Cui, X.; Fu, Q.; Yu, M.; Fei, Q.; Feng, G.; Shan, H.; Huan, Y. Colorimetric Sensor for Thiocyanate Based on Anti-aggregation of Gold Nanoparticles in the Presence of 2-Aminopyridine. Anal. Sci. 2020, 36, 1165–1169.
Lu, L.; Zhang, J.; Yang, X. Simple and Selective Colorimetric Detection of Hypochlorite Based on Anti-aggregation of Gold Nanoparticles. Sens. Actuators B 2013, 184, 189–195.
Liu, W.; Du, Z.; Qian, Y.; Li, F. A Specific Colorimetric Probe for Phosphate Detection Based on Anti-aggregation of Gold Nanoparticles. Sens. Actuators B 2013, 176, 927–931.
Duan, J.; Yin, H.; Wei, R.; Wang, W. Facile Colorimetric Detection of Hg2+ Based on Anti-aggregation of Silver Nanoparticles. Biosens. Bioelectron. 2014, 57, 139–142.
Ye, Y.; Guo, Y.; Yue, Y.; Huang, H.; Zhao, L.; Gao, Y.; Zhang, Y. Colorimetric Sensing of Copper Ions Based on the Anti-aggregation of Unmodified Silver Nanoparticles in the Presence of 1,4-Dithiothreitol. Anal. Methods 2015, 7, 566–572.
He, Y.; Zhang, X. Ultrasensitive Colorimetric Detection of Manganese(II) Ions Based on Anti-aggregation of Unmodified Silver Nanoparticles. Sens. Actuators B 2016, 222, 320–324.
Basiri, S.; Mehdinia, A.; Jabbari, A. Green Synthesis of Reduced Graphene Oxide-Ag Nanoparticles as A Dual-Responsive Colorimetric Platform for Detection of Dopamine and Cu2+. Sens. Actuators B 2018, 262, 499–507.
Bothra, S.; Kumar, R.; Pati, R.K.; Kuwar, A.; Choi, H.-J.; Sahoo, S.K. Virgin Silver Nanoparticles as Colorimetric Nanoprobe for Simultaneous Detection of Iodide and Bromide Ion in Aqueous Medium. Spectrochim. Acta A 2015, 149, 122–126.
Motahhari, A.; Abdolmohammad-Zadeh, H.; Farhadi, K. Development of A New Fluoride Colorimetric Sensor Based on Anti-aggregation of Modified Silver Nanoparticles. Anal. Bioanal. Chem. Res. 2021, 8, 79–89.
Dong, C.; Ma, X.; Qiu, N.; Zhang, Y.; Wu, A. An Ultra-Sensitive Colorimetric Sensor Based on Smartphone for Pyrophosphate Determination. Sens. Actuators B 2021, 329, 129066.
Pinyorospathum, C.; Rattanarat, P.; Chaiyo, S.; Siangproh, W.; Chailapakul, O. Colorimetric Sensor for Determination of Phosphate Ions Using Anti-aggregation of 2-Mercaptoethanesulfonate-Modified Silver Nanoplates and Europium Ions. Sens. Actuators B 2019, 290, 226–232.
Chen, L.; Fu, X.; Li, J. Ultrasensitive Surface-Enhanced Raman Scattering Detection of Trypsin Based on Anti-aggregation of 4-Mercaptopyridine-Functionalized Silver Nnanoparticles: An Optical Sensing Platform Toward Proteases. Nanoscale 2013, 5, 5905–5911.
Trisaranakul, W.; Chompoosor, A.; Maneeprakorn, W.; Nacapricha, D.; Choengchan, N.; Teerasong, S. A Simple and Rapid Method Based on Anti-aggregation of Silver Nanoparticles for Detection of Poly(diallyldimethylammonium chloride) in Tap Water. Anal. Sci. 2016, 32, 769–773.
Shellaiah, M.; Sun, K.W. Review on Nanomaterial-Based Melamine Detection. Chemosensors 2019, 7, 9.
Abdel-Lateef, M.A.; Almahri, A.; Alzahrani, E.; Pashameah, R.A.; Abu-Hassan, A.A.; El Hamd, M.A. Sustainable PVP-Capped Silver Nanoparticles as a Free-Standing Nanozyme Sensor for Visual and Spectrophotometric Detection of Hg2+ in Water Samples: A Green Analytical Method. Chemosensors 2022, 10, 358.
Mochi, F.; Burratti, L.; Fratoddi, I.; Venditti, I.; Battocchio, C.; Carlini, L.; Iucci, G.; Casalboni, M.; De Matteis, F.; Casciardi, S.; et al. Plasmonic Sensor Based on Interaction between Silver Nanoparticles and Ni2+ or Co2+ in Water. Nanomaterials 2018, 8, 488.
Rossi, A.; Zannotti, M.; Cuccioloni, M.; Minicucci, M.; Petetta, L.; Angeletti, M.; Giovannetti, R. Silver Nanoparticle-Based Sensor for the Selective Detection of Nickel Ions. Nanomaterials 2021, 11, 1733.
Liu, X.; Wang, J.; Wang, Y.; Huang, C.; Wang, Z.; Liu, L. In Situ Functionalization of Silver Nanoparticles by Gallic Acid as a Colorimetric Sensor for Simple Sensitive Determination of Melamine in Milk. ACS Omega 2021, 6, 23630–23635.
Sharma, R.; Dhillon, A.; Kumar, D. Mentha-Stabilized Silver Nanoparticles for High-Performance Colorimetric Detection of Al(III) in Aqueous Systems. Sci. Rep. 2018, 8, 5189.
Nguyen, V.-N.; Ha, J.; Cho, M.; Li, H.; Swamy, K.M.K.; Yoon, J. Recent Developments of BODIPY-Based Colorimetric and Fluorescent Probes for the Detection of Reactive Oxygen/Nitrogen Species and Cancer diagnosis. Coord. Chem. Rev. 2021, 439, 213936.
Arumugaperumal, R.; Srinivasadesikan, V.; Lin, M.-C.; Shellaiah, M.; Shukla, T.; Lin, H.-C. Facile Rhodamine-Based Colorimetric Sensors for Sequential Detections of Cu(ii) Ions and Pyrophosphate (P2O74−) Anions. RSC Adv. 2016, 6, 106631–106640.
Liu, B.; Zhuang, J.; Wei, G. Recent Advances in the Design of Colorimetric Sensors for Environmental Monitoring. Environ. Sci. Nano 2020, 7, 2195–2213.
Fang, H.-P.; Shellaiah, M.; Singh, A.; Raju, M.V.R.; Wu, Y.-H.; Lin, H.-C. Naked Eye and Fluorescent Detections of Hg2+ Ions and Cysteine via J-Aggregation and Deaggregation of a Perylene Bisimide Derivative. Sens. Actuators B 2014, 194, 229–237.
Shellaiah, M.; Thirumalaivasan, N.; Aazaad, B.; Awasthi, K.; Sun, K.W.; Wu, S.-P.; Lin, M.-C.; Ohta, N. Novel Rhodamine Probe for Colorimetric and Fluorescent Detection of Fe3+ Ions in Aqueous Media with Cellular Imaging. Spectrochim. Acta A 2020, 242, 118757.
Chen, S.; Xue, Z.; Gao, N.; Yang, X.; Zang, L. Perylene Diimide-Based Fluorescent and Colorimetric Sensors for Environmental Detection. Sensors 2020, 20, 917.
Ericson, M.N.; Shankar, S.K.; Chahine, L.M.; Omary, M.A.; Herbing, I.H.v.; Marpu, S.B. Development of Neutral Red as a pH/pCO2 Luminescent Sensor for Biological Systems. Chemosensors 2021, 9, 210.

© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license; additional terms may apply. By using this site, you agree to the Terms and Conditions and Privacy Policy.

Upload a video for this entry

Information

Subjects: Chemistry, Analytical

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Muthaiah Shellaiah

Kien-Wen Sun

View Times: 430

Update Date: 29 Dec 2022

Table of Contents

Video Upload Options

Confirm