The Advances of Hydrosol–Gel Transition-Based Sensors

The Advances of Hydrosol–Gel Transition-Based Sensors: Comparison

Please note this is a comparison between Version 1 by Haoyang Song and Version 5 by Conner Chen.

Hydrogels, as a type of three-dimensional porous material, have attracted a lot of attention in the fields of drug delivery, artificial tissue engineering, and sensing. Due to their excellent biocompatibility and high sensitivity to external stimuli, they are widely used in the development of various sensors. Among them, the sensors constructed based on the sol–gel transition of target–responsive hydrogels are particularly welcome. THerein, the status of the sensors on the basis of sol–gel transition has been presented. The types of hydrogel sensors and the analytical methods in various application scenarios are illustrated. In addition, the future trends of the sensing systems based on sol–gel transition are briefly discussed.

hydrogels
stimuli-responsive material
analytical method

1. Introduction

Hydrogels are crosslinked hydrophilic polymer porous network materials. Like other widely used porous materials such as metal-organic frameworks (MOFs) [1], hydrogels also possess attractive physical and chemical properties. A large number of hydrogels have high molecular permeability, high mechanical properties, remarkable ionic conductivity, controllable microstructures, excellent biocompatibility, and impressive stability, which show great potential in the spheres of drug delivery ^{[2][3][4][5][6][7][8][9][10]}[2,3,4,5,6,7,8,9,10], artificial tissue engineering ^{[11][12][13][14]}[11,12,13,14], sensing ^{[15][16][17][18][19][20][21]}[15,16,17,18,19,20,21], etc. Generally, hydrogels contain a large amount of water, whose weight is even a few hundred times more than that of dry hydrogel scaffolds [22]. The gel materials with various properties have different water storage capacities, leading to various applications. Hydrogels used in force change sensing often need strong mechanical properties and are often prepared by the method of physical adsorption and ultrasonic treatment ^[23][24][23,24]. Hydrogels used in biosensing are often prepared by the method of chemical covalent and require the integration of hydrogel scaffolds and recognition units, causing a highly sensitive response to various environmental stimuli. The recognition units of the hydrogels can specifically respond to external stimuli, which may be easily detected and quantified without complex operations.

There are mainly two sensing methods that apply the sol–gel transition mechanism. One is to embed the recognition units and signal molecules into the hydrogels, which are dissolved with the presence of the target substances to release the signal molecules ^[25][30]. According to the porous three-dimensional structures of hydrogels, the signal molecules can be encapsulated to avoid their rapid interaction with the external environment. For example, considering the amount of water trapped in hydrogels as a type of signal, the destruction degree of hydrogel scaffolds can be determined ^[26][31]. The other method hinders the initial signals via the target-induced formation of hydrogels. For instance, the formation of Ca²⁺-triggered hydrogels on the surface of glass carbon electrodes greatly increases the impedance signals ^[27][32]. Compared to other advanced detection methods such as gold nanoparticles colorimetric assay ^[28][33], detection based on sol–gel transition takes advantage of the simple and repetitive structure of hydrogels to allow the signal molecules to embed in but not modify the materials, which simplifies the preparing operations. It is also more convenient as a detection method that the signal amplification is achieved according to the number of embedded signal molecules. Thus, the hydrogel sensing based on sol–gel transition is particularly important in the detection of biological enzyme activity, small molecules such as organic pesticides and toxins, heavy metal ions and so on.

2. Types of Hydrogels in the Development of Sol–Gel Transition-Based Sensors

Hydrogel scaffolds with different compositions endow hydrogels with different responsive and destructive methods. According to various materials, three major kinds of hydrogels have been reported, involving DNA hydrogels, polypeptide hydrogels, and polysaccharide hydrogels. DNA hydrogels usually contain functional DNA fragments to recognize targets. Polypeptide hydrogels are usually used to detect enzymes due to their responses to different hydrolases. For polysaccharide hydrogels, besides the scaffold formed by polymers, the recognized units are usually acting as crosslinking agents.

2.1. DNA Hydrogels

For DNA hydrogels, DNA acts as a crosslinking agent or scaffold, and recognition units are often DNA-related substances, such as aptamer, DNA probe, and complementary strand. Based on the complementary characteristics of DNA double strands, the combination and separation of recognition units from scaffolds is the core of sol–gel sensing. According to different scaffold components, two major types of DNA hydrogels have been extensively studied, involving pure and hybrid DNA hydrogels. For pure DNA hydrogels, their scaffolds are composed of DNA strands with complementary bases ^[29][46]. They always have good biocompatibility and biodegradability ^[30][47]. Zhang et al. fabricated ctDNA-triggered and DNAzyme-functionalized hydrogels for visible detection of circulating tumor DNA (ctDNA) ^[31][37]. Target ctDNA triggered the rolling circle amplification (RCA), thus resulting in DNA hydrogels with G-quadruplex structures on a macroscopical scale, which was utilized as the direct detection of ctDNA. Chu selected two partially complementary strands, Pb²⁺ dependent DNAzyme strand and substrate strand, to manufacture the DNA hydrogels ^[32][48]. Pb²⁺ presenting in the sample activated the DNAzyme strands, which led to the cleavage of the substrate strands and signal output.

Hybrid DNA hydrogels are generally formed by the hybridization of modified polymers and deoxyribonucleotide chains. They always possess high mechanical strength and stability ^[30][47]. For the preparation of hybrid DNA hydrogels, two common methods were developed. The one method involves DNA modification on organic polymer chains. Acrylamide ^[20][33][34][20,35,49] and acrydite ^[35][50] are always used as monomers for polymerization. Lin developed the hybrid DNA hydrogels which were made by three different kinds of ssDNA strands combining with non-DNA polymers ^[35][50]. Their scaffolds consisted of the acrydite-modified adenosine aptamer and the acrydite-modified ssDNA partially complementary to the adenosine aptamer with the heme aptamer acting as the crosslinker. To improve the sensitivity of polymer-DNA hydrogels, Liao’s group wrapped the fluorescent quantum dots in DNA-based acrylamide hydrogel microcapsules [19]. All fluorescent quantum dots in the microcapsules can be obtained without completely collapsing the hydrogel. Another method is to modify DNA on microbeads, such as gold nanoparticles and SiO₂ nanoparticles. Jie’s group developed a versatile fluorescence strategy through DNA-SiO₂ hybrid hydrogels, which were used to detect miRNA-141 with ultra-sensitivity ^[36][34]. The target miRNA-141 firstly induced the production of large amounts of DNA S₃ by walking amplification. Then, S₃ linked with DNA hairpin H₁ which was attached to SiO₂ microspheres. After the hybridization chain reaction (HCR) of H₁ and other hairpin DNA modifying on SiO₂, DNA crosslinked hydrogels can be formed on SiO₂. Additionally, Liu used DNA-modified gold nanoparticle to fabricate hybrid hydrogels film for the biosensing system ^[37][51]. To enhance the mechanical strength of hydrogels, Ji et al. further integrated the two methods ^[38][39][52,53]. They firstly linked DNA with SiO₂ nanoparticles, then modified poly methacrylic acid on the terminal of DNA complementary strand. Finally, polymer hydrogel scaffolds were constructed outside the DNA-SiO₂ nanoparticles via DNA complementary binding.

DNA can continuously synthesize long DNA strands and form pure DNA hydrogels under the catalysis of polymerase. The operations of synthesis and signal amplification are relatively simple. Hybrid DNA hydrogels not only enrich the diversity of DNA hydrogels, provide more sensing strategies, but have better stability to adapt to complex detection environments.

2.2. Polypeptide Hydrogels

Polypeptide-based hydrogels benefit from their low immunogenicity, high bio-compatibility and biodegradability, and lower cost than DNA hydrogels, drawing great attention in the sphere of biological applications ^{[40][41][42][43]}[54,55,56,57]. On the one hand, the types of amino acids are much more than the types of nucleobases in nucleic acids, which greatly enrich the diversity of peptide chains; On the other hand, unlike nucleic acids that rely on complementary base pairing to form scaffolds, peptide chains can be linked to each other by modifying reactive groups or photosensitive groups at the end of the chain, or directly through hydrogen bonds or hydrophobic interactions.

Polypeptide hydrogels can be used for the detection of polypeptide enzymes without introducing extra recognition groups. For example, it is feasible to quantitatively detect trypsin ^[26][31] and thrombin ^[44][39] by measuring the water molecules released from gelatin and fibrinogen hydrogels due to the enzymatic hydrolysis. Ahmad and colleagues crosslinked 4-arm poly (ethylene glycol) norbornene with bis cysteine peptide to form polypeptide hydrogels, which were used to detect trace collagenase with the assistance of quartz crystal microbalance ^[45][58]. In addition, combining with DNA probes, aptamers, and other recognition units with polypeptides can enrich the types of polypeptide-based hydrogels and expand the functionality. For example, to detect cancer DNA, Jandl used the method of terminal modification to combine DNA with peptide chain, and the fluorophores entered the polypeptide hydrogels via hydrogen bonds ^[42][56].

For the good specificity of proteolytic enzymes, polypeptide hydrogels can be directly recognized by related enzymes, and the abundant enzyme-binding sites in the hydrogel scaffold facilitate the signal amplification.

2.3. Polysaccharide Hydrogels

As a kind of natural polymer material, polysaccharide hydrogels have attracted extensive attention. Alginate hydrogels as polymer backbones, have been widely used in biological analysis. Yang and co-workers noticed that it was easy to utilize alginate hydrogels with “egg-box” structures to encapsulate enzymes ^[46][59]. This method facilitated the analysis of different enzymes using the immobilized enzyme reactor (IMER) with same hydrogel structures. The species of detected enzymes depended on the substrate injected in capillary electrophoresis. Later, they provided an approach for on-line enzyme assays for penicillinase utilizing capillary electrophoresis-integrated immobilized enzyme reactors (CE-IMERs) ^[47][60]. To fabricate the CE-IMERs, the hydrosol stock suspension was prepared and injected into the pretreated capillary. The suspension contained penicillinase, sodium alginate, and calcium carbonate. Then, the penicillin G sodium salt (PG) solution was introduced into the inlet of the capillary with electrokinetics. The hydrolysis of PG by penicillinase-produced penicilloic acid (PA), which made the solution weakly acidic, thus promoting the hydrolysis of CaCO₃ to produce Ca²⁺. With the help of Ca²⁺, the sodium alginate hydrosol transited into hydrogels.

Based on Ca²⁺-triggered pH-response sodium alginate hydrogels precipitation, Ma’s group developed a cascaded signal-amplification strategy for a sandwich-type impedimetric immunosensor, which was particularly useful for the detection of prostate specific antigen (PSA) ^[27][32]. The secondary antibody was firstly connected to gold nanoparticle-CaCO₃ microspheres (AuNP-CaCO₃). When AuNP-CaCO₃ was deposited on the surface of the glass carbon electrode (GCE) by binding the target molecules to the primary and secondary antibodies, the dissolution of CaCO₃ and release of Ca²⁺ in a weakly acidic solution were induced. Consequently, Ca²⁺ was crosslinked with alginate to form insoluble alginate salt hydrogels’ precipitation on the sensing interface, which significantly increased the impedance signal. Benefiting from the cascaded signal amplification, the impedimetric immunosensor exhibited ultrahigh sensitivity. At present, although there are many types of polysaccharide hydrogels, sodium alginate hydrogels are more researched for sensing because their unique “egg-box” structure and Ca²⁺-induced gelation are suitable for sensing strategy.

3. The Methods Used for the Development of Sol–Gel Transition-Based Sensors

Various sol–gel transition-based sensors are introduced according to different sensing methods. The brief information of these sensors is put in Table 1 2.

Table 1 2.

The application and limit of detection of sensors with different using methods.

Method	Detection Application	Limit of Detection	Reference
Colorimetric assay	T-2 mycotoxin	0.87 pg/mL	^[48][61]
	Alkaline phosphatase	0.37 mU/mL	^[49][62]
	Ochratoxin A	0.005 ng/mL	^[50][63]
Fluorescence assay	Alkaline phosphatase	21.3 μM	^[51][64]
Fluorescence assay	Ochratoxin A	0.01 ng/mL	^[52][65]
Surface-enhanced Raman spectroscopy (SERS) assay	UO₂²⁺	8.38 × 10⁻¹³ M	^[53][66]
Surface-enhanced Raman spectroscopy (SERS) assay	α-fetoprotein	50 pg/mL	^[54][67]
Magnetic relaxation switching (MRS) assay	Foodborne pathogens	50 CFU/mL	^[55][45]
pH-based assay	Aflatoxin B₁	0.1 μM	^[56][38]
Weight assay by electronic balance	Hyaluronidase	0.2 U/mL	^[57][40]
	AFB₁	9.4 μg/kg	^[58][68]
	Hyaluronidase	0.35 U mL⁻¹	^[59][69]
Glucose assay by personal glucose meter	DNA adenine methyltransferase	0.001 U/mL	^[60][70]
Distance-based lateral flow assay	Organophosphorus pesticides	3.3 ng/mL	^[61][71]
	Penicillinase	2.67 × 10⁻³ mU/μL 2.67 × 10⁻² mU/μL	^[62][72]
	Glucose	1.4 mM	^[63][73]
	Pb²⁺	10 nM	^[64][74]
	Thrombin and inhibitor	16.1 mU/mL	^[44][39]