Synthesis and Modification of Magnetic Nanoparticles

Synthesis and Modification of Magnetic Nanoparticles: History

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Subjects: Chemistry, Applied

Contributor:

Soledad Carinelli

Maximina Luis-Sunga

José Luis González-Mora

Pedro A. Salazar-Carballo

Magnetic nanoparticles (MNPs) combine their magnetic properties with other interesting characteristics, such as their small size, high surface-to-volume ratio, easy handling, and excellent biocompatibility, resulting in improved specificity and sensitivity and reduced matrix effects. They can be tailored to specific applications and have been extensively used in various fields, including biosensing and clinical diagnosis. In addition, MNPs simplify sample preparation by isolating the target analytes via magnetic separation, thus reducing the analysis time and interference phenomena and improving the analytical performance of detection. The synthesis and modification of MNPs play a crucial role in adjusting their properties for different applications.

magnetic nanoparticles
biosensor
bioassay
functionalization
nanomaterial synthesis

1. Introduction

In recent decades, biosensors have gained the attention of the research community and have improved the early/high sensitivity detection of different analytes in diverse application fields. A biosensor is an electronic device used to transform a biological interaction into an electrical signal that is processed by the transductor [1]. Among these approaches, electrochemical transduction has notable advantages, particularly its remarkable simplicity and sensitivity. This sensitivity can be further enhanced by introducing (bio)catalytic labels into the transducer or into the bioreceptor-target complex, amplifying the detection signal [2]. Other benefits include their potential for miniaturization, the low cost of production, multiplexed detection, and that they do not require expensive instrumentation for read-out [3]. As a result of these advantages, today, such technology is gradually supplanting standard sophisticated techniques, and they are set to become a vital tool in healthcare and other application areas [4,5,6]. Undoubtedly, the most important biosensor, with a high impact on the health and control of diabetes, is the glucometer [7], and recent advancements have enabled the measurement of glucose from interstitial fluids just underneath the skin in continuous mode, avoiding the need to prickle the finger constantly to obtain a blood sample. However, in recent years’ new advances and applications in various fields have been achieved; for example, in the detection of volatile organic compounds (VOCs) [8,9].

Another example where biosensors will have an important impact is in the diagnosis of infectious diseases (dengue, malaria, and chikungunya fever) [10]. According to the World Health Organization guidelines, the standard methods for these infections include microbiological isolation, serological tests, and molecular techniques such as polymerase chain reaction (PCR) [11,12]. However, although PCR is the most convenient method due to its high sensitivity and relatively short analysis time, it has an elevated cost of equipment and supplies. On the other hand, it requires specialized skills personnel for the execution and interpretation of results, which makes its use in routine analysis difficult and its integration into point-of-care (PoC) systems complicated. Moreover, they are prone to generating false positives due to nonspecific amplifications.

To overcome these problems, user-friendly, low-cost, and fast methods are still needed. In this context, the development of more efficient assays, including (bio)sensors, has the potential to significantly contribute to the development of enhanced and more effective analytical tools for biomedical diagnosis [13,14]. The main drawback of conventional nonstructured biosensors is associated with the proper immobilization of the biocatalytic or biorecognition element on the surface of the transducer. Non-oriented immobilization of biomolecules may produce reusability and reproducibility problems, and low sensitivity and selectivity due to the loss of catalytic activity or biorecognition reaction, which in turn hinders the electron transfer reaction and reduces the electrochemical signal. In addition, the stability of biological materials remains one of the main challenges in the development of such devices. Several problems arise from the lack of stability and degradation of the biological elements, leading to decreased sensitivity and accuracy of the biosensor. To address these issues, several strategies can be employed: improving stabilization techniques, controlling storage conditions, selecting more robust biological elements (or engineering them for enhanced stability), and implementing routine maintenance and calibration of biosensors to compensate for any stability loss and ensure consistent performance. To address these issues, different approaches have been introduced such as nanostructuration of the transducer and the utilization of carbon-based nanomaterials, organic polymers, magnetic nanoparticles (MNPs), and magnetic beads (MBs) as solid phases for the recognition/isolation reaction [15].

Nanomaterials, with dimensions ranging from 1 to 100 nm, have played a crucial role in science, technology, and medicine over the past two decades [16]. Their unique characteristics (small size, high surface-to-volume ratio, excellent biocompatibility, biodistribution, outstanding catalytic properties, etc.) offer great possibilities for their integration into new and better devices, analytical methods, and diagnostic tools, among others. [16]. One of the main properties of such materials is that their chemical and physical properties are drastically different from those of their bulk materials and can be tailor-made for a specific task [17,18].

Magnetic nanoparticles offer several significant advantages over other nanomaterials, making them attractive for a variety of markets and applications. The most important of these are listed here:

Magnetic properties: Due to their inherent magnetism, MNPs are easily manipulated and controlled by external magnetic fields, which is advantageous in biomedical applications, such as drug delivery and hyperthermia, where localization and targeted delivery are critical.
Biocompatibility: Many MNPs, especially those coated with biocompatible materials, such as polyethylene glycol (PEG), surfactants, and proteins, exhibit low toxicity, making them suitable for biomedical applications.
Drug delivery: MNPs can be functionalized with biomolecules or specific ligands to carry drugs or therapeutic agents. Magnetic guidance of these nanoparticles can improve the efficiency and precision of drug delivery, improving therapeutic outcomes and minimizing side effects.
Biomedical imaging and diagnostics: MNPs have exceptional capabilities for high-resolution imaging and early disease detection. In this regard, MNPs serve as contrast agents in various imaging modalities such as magnetic resonance imaging (MRI), magnetic particle imaging (MPI), magnetic particle spectroscopy (MPS), multimodal PET-MRI, SPECT-MRI, and OI-MRI.
Magnetic hyperthermia: MNPs exposed to alternating magnetic fields can generate heat due to hysteresis losses. This property can be used in magnetic hyperthermia treatments for cancer, where targeted heating of tumor cells can destroy them without affecting healthy tissue.
Environmental applications: MNPs can be used as sorbents in solid–liquid extraction. Due to their magnetic nature, they can be used for separation, treatment, and remediation processes to remove pollutants and contaminants from water and soil.
Nanotechnology integration: MNPs can be easily integrated into existing nanotechnology platforms, allowing them to be used together with other nanomaterials to create hybrid systems with enhanced functionalities, including hybrid analytical methods such as magnetic bioassays and biosensors.
Catalysis: MNPs can serve as effective catalysts in various chemical reactions due to their large surface area and unique magnetic properties. They can be easily separated and reused, which makes them attractive for catalytic applications.

Due to these advantages, MNPs have been successfully implemented in multiple areas such as energy and data storage [19,20], biosensing [21], catalysis [22], bioremediation [23], neural stimulation [24], pharmacological liberation [25,26], cancer treatment [27] and clinical diagnosis [14,28]. As mentioned above, the use of magnetic sorbents significantly simplifies sample manipulation by isolating the target by applying an external magnetic field [29,30]. Similar strategies may be integrated into other analytical procedures (protein purification [31,32], remediation [33,34], chromatography [35], atomic absorption spectrometry [36], and electroanalysis [37,38]) to improve the selectivity, sensitivity, and time for results [39]. However, the numerous advantages of nanomaterials are balanced by challenges related to safety, cost, regulations, and public perception.

On the other hand, the combination of MNPs and electrochemical read-out methods has allowed the development of new analytical techniques such as enzyme-linked immunomagnetic electrochemical (ELIME) methods [40]. Here, MNPs act as nanosized supports for the immobilization of biomolecules (antibodies, aptamers, and oligonucleotides) and are used for the isolation of the target from complex matrices and its concentration before detection. The utilization of MNPs enhances the effectiveness of analyte isolation and concentration, minimizes matrix effects due to simplified washing and separation procedures, allows faster assay kinetics, improves the sensitivity and limits of detection (LOD), and reduces the time for analysis [41]. Usually, ELIME bioassays are developed in a sandwich-type format, where two specific antibodies are used (capture-Ab and labeled-Ab). The read-out is performed using differential pulse voltammetry (DPV), constant potential amperometry (CPA), linear sweep voltammetry (LSV), or similar electrochemical techniques with screen-printed electrodes (SPEs) as transducers [42]. Moreover, the use of magnetic beads in combination with magneto-biosensing strategies can avoid individual electrode surface modifications with biological elements, which simplify storage and ensure proper pre-concentration of the sample on the transducer by applying a magnet under the working electrode area.

More recently, MNPs have gained great interest in microfluidic systems due to their extensive surface area and remarkable controllable characteristics [18,43]. The integration of MNPs into microfluidic systems improves analytical performance by introducing functionalized magnetic nanomaterials into microchip devices. Sun et al. employed a magnetic nanoparticle-assisted microfluidic system (MNPAMS) for high-recovery separation of low-abundance HeLa cells [44]. The system achieved a recovery ratio of 88.6% by labeling target cells with MNPs by cocultivation and using an additional magnetic field for separation. The MNPAMS offers advantages such as ease of microstructure design and fabrication, low-cost of permanent magnets, and successful cell separation with low damage rate during recovery, enabling further research.

2. Synthesis and Modification of MNPs

2.1. Synthesis of MNPs

Depending on their applications, different synthetic and modification protocols can be used [49,50]. The choice of such methods depends on the specific requirements of the bioassay/application and the desired properties of the MNPs, such as their morphology, size, biocompatibility, stability, and magnetic properties.

Nanoparticle synthesis has become a crucial area of research, unlocking endless possibilities for innovation and applications in various fields. A large number of methods can be found in the literature, although they can be grouped under two general approaches: (1) bottom-up methods, where nanostructures are built from individual atoms, molecules, or smaller building blocks, and (2) top-down methods, where the fabrication of nanostructures starts with the manipulation of bulk materials and is then reduced to the desired nanosized structure. In addition, another interesting classification is based on the synthetic methods used: (1) physical and (2) chemical approaches. In many cases, it is feasible to associate chemical methods with the bottom-up approach and physical methods with the top-down technique. However, it is important to note that this association is not a strict rule and that, in some cases, there may be overlaps or combinations of approaches in the synthesis route. Briefly, such methods can be summarized as follows:

Physical approaches:

Vapor Condensation: In this technique, vaporized metal atoms or compounds are rapidly cooled to form nanoparticles through condensation. The size of nanoparticles depends on the temperature, pressure, and cooling rate.
Laser Ablation: High-energy laser pulses are used to vaporize a target material, and the ejected material condenses to form nanoparticles. This method allows the synthesis of nanoparticles without the need for chemical reagents.
Sputtering: In this process, energetic ions are bombarded onto a solid target material, causing the ejection of atoms and their deposition on a substrate, resulting in the formation of nanoparticles.
Ball Milling: This mechanical method consists of grinding and mixing solid materials, which results in the formation of nanoparticles due to high-energy collisions between the particles.

Chemical approaches:

Co-precipitation or chemical reduction: In this method, metal ions are dissolved in a solution and reduced to form nanoparticles by the addition of a reducing agent. The size and shape of the nanoparticles can be controlled by adjusting the reaction conditions and stabilizing agents.
Solvothermal method: This technique involves the hydrolysis and condensation of metal alkoxides or metal chlorides in a solution to produce a colloidal suspension of nanoparticles. The process allows precise control of nanoparticle composition and size.
Thermal decomposition: In this procedure, high temperatures are used to decompose precursors and produce nuclei, followed by their subsequent growth into NPs. Several factors such as temperature, solvent, reactant ratio, reflux time, and seed concentration are important to determine the size and morphology of nanoparticles.
Micro-emulsion: Nanoparticles are formed into a stable microemulsion, where the core contains the reaction precursors and the surfactants control the particle size and prevent aggregation.
Green synthesis: This approach uses plant extracts or other natural sources as reducing and stabilizing agents to produce nanoparticles. It offers an eco-friendly alternative to conventional chemical methods.

2.2. Surface Modifications of MNPs

The appropriate modification of MNPs confers them with special properties for their integration in different applications such as solubility, biocompatibility, low toxicity, stability against oxidation processes, and electrical properties. The chemical stability, hydrophobicity, catalytic properties, and biocompatibility of NPs can be improved by conjugating different organic and inorganic chemical compounds, such as silica, gold, platinum, ceria, chitosan, polyethylene glycol, polyvinyl alcohol, poly(lactic-co-glycolic acid), and polyethylenimine), during or after the synthesis of NPs [70]. Table 1 depicts a selection of works categorized by the surface modification/coating, target, and the main analytical parameters.

Table 1. Electrochemical sensor performance comparison with magnetic nanoparticles: categorized based on surface modifications/coatings.

Sensor Material	MNPs Surface Modification/ Coatings	Analyte	LoD	Linear Range	Reference
MNP/CG	chitosan	glucose	16 μM	Up to 26 mM	[76]
Fe₃O₄-chitosan-β-cyclodextrin/MWCNTs	chitosan	glucose	19.30 μM	40 μM to 1.04 mM	[77]
CNP-L/CuONP/MWCNT/Pe/GC	chitosan	triglycerides	3.2 mg/L	9.6 to 11 mg/L	[78]
Fe₃O₄@Au@PEG@HA	polyethylene glycol	brucellosis antibodies	0.36 fg/mL	10 fg/mL to 10 pg/mL	[85]
PEG-MNPs	poly ethylene glycol	glucose	3 μM	5 to 1000 μM	[86]
APBA-PEG-MNs	polyethylene glycol	Staphylococcus aureus	270 CFU/mL	100 to 100,000 CFU/mL	[87]
PPy and PPy-containing CS/Fe₃O₄	polypyrrole	glucose-6-phosphate	0.002 mM	0.0025 to 0.05 mM	[89]
Ppy NPs	polypyrrole	C-reactive protein	0.45 mg/L	0.75 to 12 mg/L	[90]
Fe₃O₄@PANI	polyaniline	catechol	0.2 nM	-	[91]
Fe₃O₄@PANI NPs	polyaniline	creatinine	0.35 nM	0.02 to 1 μM	[92]
PANI/MG	polyaniline	hydroquinone	2.94 μM	0.4 to 337.2 μM	[93]
GOx-Au-pDA-Fe₃O₄ MBNPs	polydopamine	glucose	6.5 mM	0.02 to 1.87 mM	[95]
MNPs@pDA-Ab	polydopamine	Legionella pneumophila	10,000 CFU/mL	10 to 100,000 CFU/mL	[96]
MNPs@pDA	polydopamine	H₂O₂	0.23 μM	0.5 to 30 μM	[97]
MNP-MSN@CUR@ZnO@pAbs	silica	Salmonella Typhimurium	Colorimetric: 63 CFU/mL Fluorescent: 40 CFU/mL	10² to 10⁷ CFU/mL	[99]
Silica-coated MNPs	silica	dopamine, uric acid and folic acid	12, 14 and 18 nM	1 to 30.6, 1 to 286 and 1 to 369 μM	[100]
3D mag-MoO₃–PDA@Au NS	gold	SARS-CoV-2	10 fg/mL	10 fg/mL to 1 ng/mL	[101]
gold-coated magnetic nanoparticles	gold	leukemia cells	10 cells/mL	10 to 1,000,000 cells/mL	[103]
DTSSP-AuNPs	gold	dopamine	10 nM	0.02 to 0.80 μM	[104]
MrGO@AuNPs	gold	bisphenol A	0.141 pg/mL	0.01 ng/mL to 100 ng/mL	[105]
AuNPs/BSA/Fe₃O₄	gold	glucose	3.54 μM	0.25 to 7.0 mM	[106]
Fe₃O₄@Au NPs	gold	Pb²⁺	15 pM	50 pM to 1 μM	[108]
Magnetic beads and Pt NPs	platinum	thyroid stimulant hormone	0.004 mU/L	0.013 to 12 mU/L	[109]
MPt/CS NPs	platinum	Human chorionic gonadotropin	0.039 ng/mL	-	[110]
PtNP-PAMAM-MNP/GO-CMC^w	platinum	Xanthine	13 nM	50 nM to 12 μM	[111]
SPIONs and CdTe-MPA QDs^x	QD	Escherichia coli	100 bacterial cells	100 to 400 μg/mL	[113]
NAC-CQDs^y	QD	histamine	21.15 ppb	0.1 to 100 ppm	[114]

This entry is adapted from the peer-reviewed paper 10.3390/chemosensors11100533

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