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Chapter 2.6. Gold nanoparticles for diagnostic development.

Authors: 

Khairunisak Abdul Razak, Siti Rabizah Makhsin, Nor Dyana Zakaria, and Noor Hashimah Mohd Noor

Art work
Title: 
Aftermath
Artist: 
Uli Reinhardt
Laboratories working in natural disasters face problems 
associated to isolation, lack of electricity, equipment, 
reagents, and trained personnel.
They stand close up in a throng, waiting,
a crowd whose faces have no expressions.
Tomas Tranströmer 
The Couple. 

Introduction

Nanotechnology has become a topic of interest in various fields including biomedicine. The term refers to materials that are less than 100 nm in at least one dimension. Nanotechnology has gained considerable interest in biomedicine because the materials involved have large surface areas, which affect their physical and chemical properties. Nanoparticles of various materials such as gold, iron oxide, silver, carbon nanotubes, graphene, zinc oxide, alumina, silica, and titania have been studied and used extensively in biomedicine. The most common applications are diagnostics, theranostics, drug delivery, and imaging.

Gold nanoparticles (AuNPs) are widely used in biomedical applications because they have several favorable properties: they are noble (i.e., inert); their shape and size can be easily tuned by varying the synthesis protocols; their surfaces can be easily functionalized; they easily form stable colloids; and they have low cytotoxicity. The localized surface plasmon resonance (LSPR) of AuNPs has been used in many applications because the color of the particles changes with size and shape. When AuNPs are exposed to an electromagnetic wave of a certain wavelength, their conduction electrons are driven by the electric field so that they collectively oscillate relative to the lattice of the positive nuclei, creating intense extinction peaks (extinction = absorption + scattering) at resonant wavelengths. AuNPs absorb light at a certain wavelength, which results in the emission of light of a complementary wavelength. For example, typical 40-nm gold nanospheres absorb green light at 520 nm and produce complementary red light. Therefore, the color of a colloidal solution of gold can be changed by tuning the size and shape of the gold particles. With in vivo applications, LSPR of AuNPs must take place at 700–900 nm so that light can penetrate sufficiently into soft tissue to achieve a large reduction in absorption from hemoglobin (blood) and water, and in scattering from the tissue. Normally, LSPR in the near infrared region (>700 nm) is only applicable to AuNPs with non-spherical morphologies or hollow structures such as rods, plates, multipods, stars, shells, boxes, cages, or frames. Examples of absorbed and complementary color at a certain wavelength range are given in Table 1.

Table 1. Absorbance and complementary color

Various types of AuNPs have been used in biomedical applications: nanospheres, nanorods, nanostars, bi-pyramids, and nanocages, as shown in Figure 1. With gold nanospheres, LSPR can be tuned by changing the size of the nanoparticles. Therefore, the use of gold nanospheres depends on the size of the nanoparticles [1]. For example, imaging applications require a particle size of 2–10 nm, whereas diagnostic kit applications require 30–40-nm particles. Gold nanospheres are relatively well established in terms of synthesis technologies and properties, and have several advantages: it is easy to synthesize them, they are stable as colloids, it is easy to functionalize their surfaces without the need for additional chemicals such as crosslinkers, and they have a long lifetime. However, LSPR can only be tuned up to 600 nm with gold nanospheres.

Gold nanorods have two surface plasmon resonance modes: a longitudinal surface plasmon (LSP) band at 700–1300 nm and a transverse surface plasmon (TSP) band at 520 nm [2]. LSP is dependent on the length of the rods, whereas TSP is dependent on their width. Therefore, the surface area of the gold nanorods can be tuned by changing the dimension of the nanorods. However, nanorods have several drawbacks: they are difficult to synthesis in bulk, it is difficult to obtain stable colloidal suspensions of nanorods, and they require surface functionalization prior to conjugation with biomolecules. Gold nanostars have been actively studied in recent years for use in cancer treatment. Nanostars are excellent radiators because electrons on the spiky surface structure are able to capture light and produce higher temperatures compared with gold nanospheres [3]. Moreover, gold nanostars absorb lower-energy light, which is safer for healthy cells caught in the beam. Nanocages are another type of gold nanoparticle [4]. Gold nanocages have attracted interest especially in drug delivery and theranostic applications, whereby drugs can be loaded and delivered to the target site efficiently owing to their hollow structure. Among the various particle shapes, nanospheres have been widely used in diagnostic applications because they are easily synthesized, easily conjugated, and have longer lifetimes. Therefore, in this chapter we will focus our attention on gold nanospheres.

Figure 1. Various types of gold nanoparticles (AuNPs); plasmon resonance varied with size and shape: (a) nanoparticles (b) nanorods [1], (c) nanostars [2], (d) bipyramids [3], (e) nanocages [4].

In diagnostics, gold nanoparticles have been used in electrochemical systems or have been used in applications based on their optical properties. Electrochemical sensors depend on the electrical properties of biofunctionalized electrodes in the presence of a sample analyte. Electrochemical sensors can be divided into amperometric, impedimetric, potentiometric, and conductometric. Optical sensors depend on changes in color or in surface plasmon resonance. Optical sensors can be divided into colorimetric, surface plasmon resonance, fluorescence quenching, and biobarcode sensors. In this chapter, we discuss bioconjugation, the latest developments in the synthesis of nanoparticles, and biosensors based on electrochemical and optical sensors.

Gold nanoparticles

AuNPs can be synthesized into various shapes including spheres, rods, quantum dots, multipods, cubes, nanoclusters, nanofibers, stars, or hollow structures such as shells, tubes, cages, or boxes [5, 6]. Figure 2 shows the general formation of AuNPs using a common chemical reduction method. By carefully designing the size, shape, and composition of the AuNPs, the proportion of light scattering relative to light absorption can be optimized for the intended application.

Figure 2. Synthetic route for the formation of gold nanoparticles (AuNPs) of different shapes [6].

2.1 Gold nanoparticles and its properties

AuNPs are defined as stable colloid solutions of clusters of Au atoms with particle dimensions in the range 1–100 nm [7]. On this scale, AuNPs show different physicochemical characteristics than bulk gold. The most obvious difference is the color change from yellow to ruby red when bulk Au is converted to nanoparticulate Au. The ruby red color of AuNPs can be explained by a theory called “surface plasmonics”. According to this theory, when clusters of Au atoms encounter the electromagnetic field of the incoming light, the surface free electrons present in the conduction band of the AuNPs (six electrons) oscillate, creating a plasmon band that has an absorption peak (500–540 nm) in the visible region.

The physical properties of AuNPs are dependent on several factors including their size, shape, the distance between particles, and the nature of the stabilizer used to prevent their agglomeration. According to Mie theory, surface plasmon resonance (SPR) is absent from AuNPs that are smaller than 2 nm and larger than 500 nm [8]. Au nanorods have two absorption bands: a longitudinal-wavelength band at 550–600 nm and a transverse-wavelength band at 520 nm. The longitudinal-wavelength band is very sensitive to ratio changes in the Au nanorods, whereby the absorption region moves from visible to near-infra red (NIR). This unique optical property makes Au nanorods very useful in NIR ray therapy [9] and enhanced Raman scattering of absorbed biomolecules [10]. The presence of six free electrons in the conduction band of AuNPs makes them potential candidates to bind with thiols and amines [11]. Thus, AuNPs are easily tagged with various proteins and biomolecules that are rich in amino acids, and are therefore useful for important biomedical applications including targeted drug delivery, cellular imaging, and biosensing [12-15]. Moreover, AuNPs with high electron densities are excellent contrast enhancement agents for tumor detection. Details about the various types of AuNPs and their unique properties are presented in Table 2.

Table 2. General properties of gold nanoparticles (AuNPs).

Synthesis methods

AuNPs have been used in chemical, biological, and medical applications owing to their oxide-free surfaces, bioconjugation properties, favorable biocompatibility, and unique optical properties. Specifically, AuNPs have been extensively used in colorimetric biosensors, drug delivery, cancer imaging, and cancer therapies owing to their optical activity [32]. AuNPs have the potential to help establish specific beneficial processes and achieve selectivity within biological settings. These applications have sparked great interest in the development of synthetic methods for preparing different Au-based nanostructures. General synthesis methods for AuNPs are briefly described in this section.

Various methods have been established for the synthesis of AuNPs. Two main approaches can be used for synthesizing AuNP colloids: the “top-down” approach and the “bottom-up” approach. In the “top-down” approach, the NPs are produced based on the dispersion of larger particles. In contrast, in the “bottom-up” approach, which is sometimes known as the reduction method, NPs are produced by condensing smaller units (atom-sized) to larger units (molecule-sized) [33, 34]. Most commonly, Au (III) salt is reduced to Au (0) to form an activated species, either in a single step or via an Au (I) intermediate followed by reduction to Au (0). These activated Au (0) species are thermodynamically unstable; they rapidly aggregate to form nuclei and eventually the desired AuNPs. This is typically achieved not only by altering the relative growth rates of different facets by the selective localization of surface-modifying or capping agents, but also by the modulation of nucleation and reaction parameters such as the type of reducing agent [35-39], the reaction temperature [40-42], the reagent concentration [43, 44], and the pH [1, 45, 46]. The typical methods of synthesizing AuNPs are summarized in Table 3. These methods have been widely used in the synthesis of NPs, especially AuNPs.

Table 3. Methods of synthesizing gold nanoparticles (AuNPs).

Selected strategies for the conjugation of biomolecules to gold nanoparticles

Noble metal NPs are attractive as biolabeling and bioimaging materials for use in biomedicine because they have unique optical properties. The most important purpose of in vitro biolabeling and bioimaging is the selective and specific observation of interesting molecules, substrates, or regions. Bioconjugation of noble metal NPs has been used to create novel complexes with unique characteristics for targeting specific molecules. The dimensions of the metal NPs are similar to those of biomolecules such as proteins (enzymes, antigens, antibodies) or deoxyribonucleic acid (DNA), with dimensions in the range 2–20 nm [7]. The bioconjugation of noble metal NPs mainly takes place through one of four major approaches (Figure 3): (1) electrostatic adsorption, (2) conjugation of the ligand on the NP surface, (3) conjugation to a small cofactor molecule that the protein can recognize and bind to, and (4) direct conjugation to the AuNP surface [86-88]. The details are summarized in Table 4.

Figure 3. The main approaches to the bioconjugation of gold nanoparticles (AuNPs). (a) electrostatic attachment of a protein, (b) covalent attachment to the AuNPs ligand, (c) attachment of a protein cofactor to the AuNPs, and (d) direct linkage of an amino acid to the AuNPs core.

 

Table 4. The main strategies for bioconjugating gold nanoparticles (AuNPs) with biomolecules

Applications of gold nanoparticles in diagnostics

Electrochemical

Electrochemical biosensors created by coupling biological recognition elements with electrochemical transducers—based on, or modified with, AuNPs—have played an increasingly important role in biosensor research over the last few years. The great potential of these bioelectroanalytical devices depends on the unique properties of AuNPs [99, 100]. For example, AuNPs present stable surfaces for the immobilization of biomolecules that retain their biological activities (probably owing to enhanced freedom of orientation), which is extremely useful when preparing biosensors. Moreover, AuNPs permit direct electron transfer between redox proteins and bulk electrode materials, which facilitates electrochemical sensing without the need for electron-transfer mediators [101]. Various characteristics of AuNPs such as their high surface-to-volume ratio, their high surface energy, their ability to reduce the distance between proteins and metal particles, and their ability to act as an electron-conducting pathway between prosthetic groups and the electrode surface may facilitate electron transfer between redox proteins and the electrode surface [102]. Moreover, AuNPs provide useful interfaces at which the redox reactions of molecules such as hydrogen peroxide, oxygen, or NADH, which are involved in biochemical reactions of analytical significance, can be electrocatalyzed. Finally, a major practical advantage of the use of AuNPs is that their size and surface morphology can be controlled experimentally in an easy manner by adjusting the preparation conditions.

Electrochemical detection methods have several advantages such as easy operation, low cost, and high sensitivity; moreover, they use simple instruments and are suitable for portable devices [103]. Signal amplification and noise reduction to obtain a lower detection limit are critical for the development of successful electrochemical immunoassays [104]. Competitive/sandwich-type immunoassays with high specificity and sensitivity using pairs of matched aptamers and are used extensively for the investigation of low-level contamination [105].

The bioactivity, stability, and quantity of the biological recognition elements immobilized on the electrode are important factors in bioelectrochemistry. The adsorption of biomolecules directly onto the naked surfaces of bulk materials frequently results in their denaturation and a loss of bioactivity. AuNPs are excellent candidates for the immobilization platform. When biomolecules are adsorbed onto the surfaces of AuNPs they retain their bioactivity and stability because of the biocompatibility and high surface free energy of the AuNPs [106]. Compared with flat gold surfaces, AuNPs have a much higher surface area, facilitating loading of a larger amount of protein, and they are potentially more sensitive. Thus, a number of laboratories have explored the potential of AuNPs for biomolecular immobilization. For example, Lai et al. developed a new and disposable electrochemical immunosensor for the detection of alpha-fetoprotein (AFP) as a model analyte, with sensitivity enhancement based on enzyme-catalyzed silver deposition onto irregular-shaped gold nanoparticles (ISGNPs). The analytical procedure for the electrochemical immunoassay of AFP is schematically depicted in Figure 4. The assay was carried out using a sandwich-type immunoassay protocol with ISGNP-labeled anti-AFP antibodies conjugated with alkaline phosphatase (ALP–Ab2) as detection antibodies, with a detection limit of 5.0 pg·mL−1 AFP [104]. Many similar studies have been reported for the construction of biosensors based on the immobilization of different elements with AuNPs, such as amino acids [107], antioxidative vanillin [108], lysozyme [109], cholesterol [110, 111], and human serum albumin [112]. Selected AuNP-based electrochemical detection methods are listed in Table 5.

Figure 4. Schematic illustration and measurement principles of the electrochemical immunoassay using enzyme-catalyzed silver deposition on irregular-shaped gold nanoparticles (ISGNPs).

 

Table 5. Selected gold nanoparticle (AuNP)-based electrochemical detection methods.

In addition to enzyme [119], antigen, or antibody immobilization, electrochemical DNA biosensors based on the immobilization of oligonucleotides [120] with AuNPs have also been studied extensively. To bind to the AuNPs, the oligonucleotides generally require modification with special functional groups that can interact strongly with the AuNPs. Thiol groups are usually used for DNA and gold linkages [121, 122]. Jin et al. immobilized thiol-modified probe oligonucleotides at the 5ʹ-phosphate end on an AuNP-modified electrode surface, as shown in Figure 5 [123]. Owing to the high surface-to-volume ratio of the AuNPs, hybridization of the target DNA increased dramatically. Other functional groups were also investigated.

Figure 5. The preparation of a DNA biosensor and the detection of DNA hybridization [123].

AuNPs-based immunosensors for food analysis can be also found in the literature. An interesting work describes the detection of Salmonella enterica subsp. enterica serovar Typhimurium LT2 (S) in skimmed milk samples. A disposable immunosensor for Salmonella enterica detection using a magneto-immunoassay and AuNPs as label for electrochemical detection is shown in Figure 6 [114].

Figure 6. Schematic magneto immunoassay for Salmonella detection. (a) Principle of the assay. The first step comprises incubation of Salmonella (S) with magnetic beads (MBs) modified with primary antibodies specific to the bacteria (pSAb) (MBs-pSAb). During this step, Salmonella is captured and it remains in the MBs-pSAb/S conjugate. During the second step, the MBs-pSAb/S conjugate is captured by the application of a permanent magnetic field, and is washed. The third step involves the incubation of the MBs-pSAb/S conjugate with gold nanoparticles (AuNPs) modified with secondary antibodies (sSAb-AuNPs), capture using a permanent magnetic field, and washing. (b) Electrochemical detection of MBs-pSAb/S/sSAb-AuNPs captured onto screen-printed carbon electrodes (SPCE) using a magnetic field (Step 5) by the DPV technique. Other experimental conditions are described in the text.

3.2 Optical

AuNPs have been widely used in diagnostic applications in recent years because they are easy to synthesize, their surfaces are easily modified, they have advantageous and tunable optical properties, and excellent biocompatibility, which makes them very suitable for biomedical applications [124, 125]. The optical properties of AuNPs arise from the interaction between light and electrons on the AuNP surfaces. At a specific wavelength (frequency) of light, collective oscillation of the electrons on the AuNP surfaces causes a phenomenon called surface plasmon resonance (SPR), which results in the strong extinction of light [126]. The particular wavelength of light at which SPR occurs is strongly dependent on the size, shape, surface, and agglomeration state of the AuNPs [127, 128]. The unique light-scattering properties of AuNPs provide a wide range of opportunities for their application in interfacing biological recognition events with signal transduction, and in the design of multiple biosensing devices. The absorbance and scattering properties of AuNPs depend on their size [128, 129]. AuNPs of less than 20 nm exhibit SPR but result in negligible scattering, and are therefore used widely for the colorimetric detection of analytes. Large AuNPs (20–80 nm) cause relatively high scattering, making them more suitable for biomedical applications [130].

Optical sensors usually depend on optical changes such as the appearance or disappearance of colors or SPR. AuNPs that are linked to bioreceptors provide labeled conjugates that can be used to optically follow biorecognition events at biosensor surfaces. The most prominent detection techniques are based on the interaction between AuNPs and light, and they facilitate the direct analysis of numerous components in samples. Various optical methods have been employed to detect AuNPs such as surface-enhanced Raman scattering, or colorimetric, scanometric, fluorescence, and electrochemical techniques [131-133]. The unique properties of AuNPs have permitted the development of novel AuNP-based assays for molecular diagnostics, which promise increased sensitivity, specificity, and multiplexing capability.

3.2.1 Calorimetric

The colorimetric assay offers a simple and inexpensive way of diagnosing disease [126]. AuNPs colorimetric assays take advantage of the fact that analytes induce aggregation events that result in measurable changes and shifts in the AuNPs SPR bands [134]. AuNPs have an extraordinarily high extinction coefficient, originating from their inherent plasmonic properties. Their optical properties are strongly dependent on the distance between particles, and aggregation causes a massive shift in the extinction spectrum that is manifested as a change in the color of suspensions from red to purple [135, 136]. The clearly distinguishable color shifts produce a very simple sensor readout that can often be discerned by the naked eye. Most AuNP colorimetric sensors are designed in such a way that binding of an analyte causes particle aggregation and a consequent colorimetric response.

Lu et al. were the first to report a colorimetric biosensor for detecting breast cancer cell lines using oval-shaped AuNP-based nanoconjugates (2010) [137]. They reported that when multifunctional oval-shaped AuNPs are mixed with the breast cancer SK-BR-3 cell line, a distinct color change occurs, and the two-photon scattering intensity increases by approximately 13 times. The colorimetric two-photon scattering assay for the highly selective and sensitive detection of SK-BR-3 breast cancer cells at a level of 100 cells/mL was developed using a multifunctional (monoclonal anti-HER2/c-erb-2 antibody and S6 RNA aptamer-conjugated), oval-shaped AuNP-based nanoconjugate, as shown in Figure 7. Lu et al. also claimed that their assay was highly sensitive to the SK-BR-3 cell line and was able to distinguish it from other breast cancer cell lines that express low levels of HER2. Commonly, in the early stages of cancer development, cancer cells have a very low density of target membrane proteins that facilitate the recognition of specific cancer cells [138]. Multivalent binding was carried out in this case to increase sensitivity and selectivity. The large surface areas of AuNPs have made it easier to incorporate several recognition elements on the same surface.

Figure 7. Schematic representation of monoclonal anti-HER2 antibody and S6 RNA aptamer-conjugated oval-shaped gold nanoparticles for detecting the SK-BR-3 breast cancer cell line [137].

Saxena et al. (2012) [139] have reported a colorimetric assay for detecting Bluetongue virus (BTV) disease that could be used to develop a pen-sized diagnostic test. Bluetongue disease is a non-contagious, arthropod-transmitted viral disease that affects domestic and wild ruminants [140]. The disease is characterized by hyperthermia, petechia on the buccal membrane, glossal cyanosis, coronitis, abortion, and congenital abnormalities in newborn lambs. The common techniques used to detect the disease, such as agar-gel immunodiffusion, counter-immunoelectrophoresis, and complement fixation, are time-consuming and have low specificity. The researchers identified the immunodominant regions of the VP7 protein of BTV and synthesized them in multiple antigenic peptide (MAP) formats with cysteine at the C-terminal of the lysine mosaic. This resulted in a highly ordered conformation and enzyme-linked immunosorbent assay (ELISA) reactivity. Finally, they coated the MAP peptides onto the AuNPs, which could then be used to detect BTV-specific antibodies in the sera using a spot test. The spot test for the detection of BTV antibodies in the serum is based on the principle of AuNPs SPR. AuNPs have free electrons on their surface, which, when excited by discrete photons/light energy, produce an oscillatory wave that is referred to as surface plasmon. Surface plasmon of AuNPs is generally due to absorption of green light at 520 nm. When nanoparticles aggregate, the wavelength at which light is absorbed increases (known as red shift) owing to the electrodynamic interaction generated by mutual dipoles and to plasmon resonance coupling. This is the explanation for the change from pink to violet after aggregation. The application of antigenic peptides has minimized the risk of handling infectious organisms, and is highly specific because it makes use of an antigenic determining region of the VP7 protein of BTV.

Recently, further research has been carried out into colorimetric sensors using AuNPs for diagnostic applications. Jeon et al. (2013) [141] have developed a diagnostic technique for malaria detection that is simple, rapid, and highly sensitive. The proposed method is based on the interaction between Plasmodium lactate dehydrogenase (pLDH), a biomarker for malaria, and a pL1 aptamer. The cationic polymers poly(diallyldimethylammonium chloride) (PDDA) and poly(allylamine hydrochloride) (PAH) are used to aggregate the AuNPs. It is possible to observe a change in color from red to blue that is dependent on the concentration of pLDH. It is possible to detect pLDH proteins at low detection limits using this aptasensor. The limit of detection for the naked eye is 1 nM pLDH for the PDDA and PAH biosensor. The aptasensor is considered a major advancement because it can be used to diagnose malaria simply and rapidly.

In the same year, a colorimetric sensing strategy using AuNPs and a paper assay platform was developed for tuberculosis diagnosis by Tsai et al. (2013) [142]. Unmodified AuNPs and single-stranded detection oligonucleotides were used to achieve rapid diagnosis without the need for complicated and time-consuming thiolated or other surface-modified probe preparation. To eliminate the use of sophisticated equipment for data analysis, the color variance for multiple detection results was simultaneously collected and concentrated on cellulose paper, with the data readout transmitted for cloud computing via a smartphone. The results showed that the 2.6 nM Mycobacterium tuberculosis target sequences extracted from patients were detected easily, and the turnaround time from DNA extraction from the clinical samples was approximately 1 h. After mixing with the AuNPs colloid and triggering with sodium chloride solution, the unmodified single-stranded deoxyribonucleic acid (ssDNA) detection sequences were directly hybridized with the extracted double-stranded DNA (dsDNA) from the tuberculosis patients or healthy individuals, without the need for complicated AuNP probe preparation. If the target DNA sequences were absent, the detection ssDNA sequences were absorbed onto the AuNP surfaces and protected the unmodified AuNPs from aggregation in the high-concentration salt solution. The ssDNA detection sequences were hybridized with the target DNA when the target DNA sequences were present, and the unprotected AuNPs colloid turned from red to blue. The technique uses AuNPs in a catalytic reaction to amplify the colorimetric signal in a more compact paper assay format; it requires smaller samples for testing and a lower concentration of M. tuberculosis. Other uses of AuNPs in colorimetric detection for diagnostics are summarized in Table 6.

Table 6. Current uses of gold nanoparticles (AuNPs) in colorimetric detection for diagnostics

Surface Plasmon Resonance

SPR is a phenomenon that occurs on a metal surface when a beam of light strikes it at a particular angle. Metal nanoparticles, such as gold and silver, exhibit localized surface plasmon resonance (LSPR), which results in strong light scattering and the appearance of intense surface plasmon absorption bands. It is well known that the size and thickness of materials on the metal surface have an important influence on SPR, and cause a gradual reduction in the intensity of the reflected light [126, 136]. By measuring the sensitivity of SPR to the refractive index of the surrounding medium on the metal surface, the adsorption and scattering of the molecules on the metal surface and their specific targeted ligands can be measured accurately [147]. Nowadays, the most common and fundamental application of biosensing SPR instruments is the determination of affinity parameters for biomolecular interactions. SPR is useful not only for measurements involving the real-time kinetics of ligand–receptor interactions, but also for lead compound identification in pharmaceutical drug development, the detection of small molecules, DNA hybridization, the investigation of enzyme–substrate and antigen–antibody interactions, antibody characterization, and label-free immunoassays.

The general principle behind LSPR-based sensors is the wavelength shift in the LSPR spectrum arising from local dielectric changes caused by analyte adsorption. LSPR assays have been conducted both in the solution phase [148] and on surfaces coated with nanoparticle monolayers [149, 150]. The maximum absorption due to LSPR is red-shifted when AuNPs functionalized with monoclonal antibodies interact with analytes. Moreover, the wavelength shift is proportional to the quantity of ligands. Recently, Huang and El-Sayed (2013) [124] reported a novel SPR sensor for the sensitive detection of concanavalin A (ConA) as a model protein. In this sensing platform, dextran (Dex)-capped AuNPs (Dex-AuNPs) were initially synthesized in one pot and the product was used as an amplification reagent. After deposition of graphene oxide (GO) on the SPR gold film, phenoxy-derivatized dextran (DexP) was assembled on the GO-modified gold chip surface through π–π interaction. The resultant GO/DexP sensing interface specifically captured ConA, which further reacted with the Dex-AuNPs through the specific interaction between ConA and Dex, forming a sandwich configuration. The sandwich SPR sensor was able to detect ConA in the range 1.0–20.0 μg•mL-1 with a detection limit of 0.39 μg•mL-1; compared with the direct assay format, the sandwich SPR sensor led to a 28.7-fold improvement in sensitivity.

In the same year, Lee et al. (2013) [151] reported the development of a highly sensitive label-free immunosensor for the detection of HIV-1 based on the LSPR method. A uniform nanopattern of circular Au dots (10–20 nm) was fabricated on an indium tin oxide (ITO)-coated glass substrate by a simple electrochemical deposition method. The surface of the Au nanopattern was modified with HIV-1-neutralizing gp120 monoclonal antibody fragments. The modified substrate was used to quantitatively measure various concentrations of HIV-1 particles based on the longitudinal wavelength shift in the UV–Vis spectrum that results from changes in the local refractive index induced by specific antigen–antibody recognition events.

Furthermore, AuNPs have been used to enhance propagating SPR spectroscopic signals to increase sensor sensitivity. The signal amplification can be explained by the electronic coupling interaction between the propagating surface plasmon and the localized surface plasmon of the AuNPs, and depends on various factors such as size, shape, and the distance from the metal generating SPR. Based on these principles, El-Sayed et al. (2005) [152] reported that anti-epidermal growth factor receptor (EGFR) antibody-conjugated AuNPs can be used to detect living oral epithelial cancer cells in vitro. The technique is based on SPR scattering images and SPR absorption spectra from both colloidal AuNPs and conjugated AuNPs-anti-EGFR antibody. The conjugated AuNPs-anti-EGFR specifically and homogeneously binds to the surface of the cancer cells with 600% greater affinity than to the non-cancerous cells. This specific and homogeneous binding gives a relatively sharp SPR absorption band with a red-shifted maximum compared with that observed in non-cancerous cells.

Recently, Bai et al. (2013) [153] developed a sensor based on an SPR platform for the real-time detection of subnanomolar thrombin. The proposed aptasensor was fabricated using a novel aptamer/thrombin/aptamer-AuNPs sandwich-enhanced SPR sensor. AuNPs were introduced to this platform for signal amplification, which resulted in high sensitivity for thrombin detection. The sensor also showed good selectivity for thrombin and was not affected by other proteins such as bovine serum albumin (BSA) or lysozyme. Other uses of AuNPs in SPR diagnostic assays are summarized in Table 7.

Table 7. Use of gold nanoparticles (AuNPs) in surface plasmon resonance (SPR) assays for diagnostic kits.

3.2.3 Fluorescence quenching

An alternative approach to optical detection in biorecognition processes consists of the use of AuNPs as local quenchers of fluorescence dyes. With the ability to quench excited molecular states, AuNPs function as effective photoluminescence (PL) quenchers in fluorescence-based sensors [158]. When the analyte binds to a spacer, the fluorophore is near to the surface of the AuNP and fluorescence can be efficiently quenched by energy transfer mechanisms. In the absence of the analyte, the spacer molecule is extended and the fluorescence is not quenched. The higher the concentration of the analyte in solution, the lower the intensity of the fluorescence signal [159]. Typically, fluorescence quenching takes place either by Förster resonance energy transfer (FRET) or nanometal surface energy transfer (NSET). Further research has shown that small AuNPs (1.4 and 3 nm) have negligible LSPR absorption and quench fluorescence mainly via an NSET mechanism, whereas larger AuNPs (15 and 80 nm) depend on a FRET mechanism [160, 161]. The critical parameter for a fluorescent sensor using AuNPs is the change in distance between the fluorophore and the AuNPs, which can be accomplished by the biological recognition event between the biomolecule modified-AuNPs and a given fluorophore probe.

Mayilo et al. (2009) [162] reported the first homogeneous sandwich immunoassay with AuNPs as fluorescence quenchers. The sandwich assay is designed for the detection of cardiac protein troponin T (cTnT) by its simultaneous interaction with two different antibodies, one attached to AuNPs, and the other labeled with fluorescent dyes. The researchers demonstrated the working principle of the assay using time-resolved fluorescence spectroscopy to determine the quenching efficiency of the AuNPs. Figure 8 shows the principle of operation of the sandwich immunoassay. Upon addition of cTnT, the M11.7-AuNPs and the M7-dye bind to different positions of the cTnT molecules forming sandwich assemblies. In the absence of cTnT, there is no interaction between the dye-label and the AuNP-conjugated antibodies, and the dye molecules emit fluorescence. In the presence of cTnT, the two antibodies become attached to it bringing the fluorescent dyes into close proximity with the AuNPs; the fluorescence emission is quenched.

Figure 8. Principle of operation of the sandwich test for cardiac protein troponin T (cTnT).

Subsequently, Guirgis et al. (2011) [163] developed a simple and sensitive immunoassay that successfully detects malaria antigens in the infected blood. This homogeneous assay is based on the fluorescence quenching of cyanine 3B (Cy3B)-labeled recombinant Plasmodium falciparum heat shock protein 70 (PfHsp70) upon binding to AuNPs functionalized with an anti-Hsp70 monoclonal antibody. Upon competition with the free antigen, the Cy3B-labeled recombinant PfHsp70 is released to solution resulting in an increase in fluorescence intensity. The researchers have also compared two types of AuNP-antibody conjugates to be used as probes: one obtained by electrostatic adsorption of the antibody onto the AuNPs surface and the other by covalent bonding using protein cross-linking agents. Electrostatic adsorption of the antibody-AuNPs surfaces generated conjugates with increased activity and linearity of response compared with cross-linked antibodies. The fluorescence immunoassay was successfully applied to the detection of antigens in malaria-infected human blood cultures at a 3% parasitemia level, and is assumed to be capable of detecting parasite densities as low as 1,000 parasites per μL.

In the same year, Ganbold et al. (2011) [30] developed a system for the detection of a single-nucleotide polymorphism of H1N1 virus DNA. The effects of AuNP aggregation were investigated for the discrimination of single point mutations through the hybridization of oligonucleotides modified with a fluorescent Texas Red dye. The sequences of oligonucleotides were designed to detect the H1N1 virus gene. Single-base mismatch detection of the different adsorption propensities of oligonucleotides was achieved using fluorescence quenching and the surface-enhanced Raman scattering (SERS) properties of the dye. The researchers found that the addition of perfectly matched double-stranded DNA (pmdsDNA), modified with Texas Red in the suspension of citrate-reduced AuNPs, increased fluorescence recovery more than either single-base mismatched double-stranded DNA (sbmdsDNA) or single-stranded DNA (ssDNA).

Biobarcode

The detection of biomolecules, particularly proteins and nucleic acids, is of interest in life sciences and diagnostic medicine. ELISA is usually used to detect proteins, and is often applied to developing a clinical methodology [164]. ELISA is an immunoassay technique based on colorimetric end-point measurement. Another very sensitive antigen-detection system called immuno-polymerase chain reaction (I-PCR) is based on ELISA. I-PCR can detect a target with a sensitivity of 10–100 copies/sample, and is particularly useful for nucleic acids [165]. PCR has been extensively used in clinical medicine, biology, and environmental chemistry owing to its high sensitivity and good quantification capabilities. However, further improvements to the sensitivity and convenience of I-PCR may be limited by the poor DNA-to-detection antibody ratio (usually 1:1) [166]. Nanostructures are ideal platform materials for biomolecular recognition and detection because they have favorable physical and chemical properties such as morphology, size, and composition. Specifically, DNA-functionalized NPs have been used to develop a novel nucleic acid and protein signal detection platform called the biobarcode assay (BCA), which has significant advantages over current techniques [166]. The BCA is highly sensitive. In the benchtop format, the BCA achieves low attomolar sensitivity for protein analytes: up to five orders of magnitude lower than analogous ELISA technology. It has also exhibited high zeptomolar sensitivity for nucleic acid targets, surpassing the I-PCR amplification method, which requires thermal cycling. The BCA is also highly specific and capable of extensive multiplexing. Therefore, the BCA offers several unique diagnostic opportunities including early disease detection, monitoring of disease recurrence, and the possibility of simultaneous multiplexed analysis of a panel of disease markers [167].

In 2003, Nam et al. established a novel ultra-sensitive immunoassay in the form of a nanoparticle-based BCA for the identification of protein targets (a prostate-specific antigen (PSA)) in an homogeneous sandwich assay [166]. PSA is a biomarker protein for prostate and breast cancer detection [166, 168]. The AuNPs used in the method were functionalized with short DNA oligonucleotides and antibodies to form a probe, which provided a large DNA-to-antibody ratio (usually 100–300:1) [169, 170]. This high ratio is the main reason BCAs exhibit ultra-sensitive detection (up to attomolar level) compared with other methods. This biobarcode technique is useful because thiol-capped DNA and antibodies are easy to label or conjugate to the surface of AuNPs [171]. Moreover, AuNPs are capable of supporting hundreds or thousands of biobarcodes, depending on the size of the particle, and therefore act as surrogate targets and amplifying agents [167].

The main components involved in developing BCA are illustrated in Figure 9 (a). A typical BCA combines microfluids with one of two types of particle that are associated with antibodies and DNA to achieve extremely high sensitivity. Micron-sized magnetic beads called magnetic microparticles (MMPs) are one of those types of particle; they are adorned with antibodies (abs) (typically monoclonal abs) as recognition elements that are specific to the protein target of interest. The other type of particle is a AuNP functionalized with both target-capture antibodies (typically polyclonal abs) and hundreds of barcode-capture oligonucleotides that are hybridized to barcode DNA [172]. The general concept of BCA is similar to an ELISA: the protein target is captured in a sandwich format. The MMPs are added to the solution containing the target. Bound analytes are detected using a second set of antibodies that are labeled with AuNPs. DNA is then isolated and detected using a combination of AuNPs and silver deposition, and magnetic separation is used to concentrate the target protein and remove all unwanted components from the assay test solution.

A typical experimental procedure for BCA is demonstrated in Figure 9 (b). An aqueous solution of MMP probes is mixed with an aqueous solution containing a target (protein or oligonucleotide molecules, e.g., DNA). After incubation, the MMPs (functionalized with polyclonal ab, and captured through interaction with the analyte of interest) are concentrated using a magnet. The supernatant is removed, and the MMPs are resuspended. This process is repeated several times to wash the MMPs. The AuNP probes (functionalized with polyclonal ab and DNA) are then added to the assay solution. The AuNPs merge with the target immobilized on the MMPs, and provide DNA for signal amplification and protein identification. After appropriate washing of the MMPs-AuNPs sandwich, they are resuspended in pure water at elevated temperature to dehybridize barcode DNA strands from the nanoparticle probe surface. Dehybridized barcode DNA is then separated and collected from the probes with the applied magnetic field, and the barcodes are detected using either fluorescence or a scanometric method.

Figure 9. (a) Overview of components, and (b) typical biobarcode procedure [172, 173].

Details about the use of AuNPs as BCA probes are summarized in Table 8. In general, the application of BCA highlights an interesting situation, in which it is possible to detect a disease marker at a much lower concentration than has traditionally been associated with “disease-positive” patients [174]. The biobarcode method offers several advantages over current protein detection methods, as described below.

The target-binding portion of the BCA is homogeneous. Therefore, large quantity of MMPs can be added to the reaction vessel to facilitate binding between the detection antibody and the target analyte. Homogenous mixing makes BCA faster than heterogeneous I-PCR systems and can increase sensitivity, because the capturing step is more efficient [175].

The AuNP bar facilitates a high DNA-to-labeling Ab ratio that can substantially increase assay sensitivity [171].

BCA removes the need for complex conjugation chemistry for binding DNA to the labeling Abs. Barcode DNA is bound to the AuNPs probe through hybridization at the start of the labeling reaction, and is liberated for amplification or direct detection with a simple wash step. Because the labeling Ab and DNA are present on the same particle, there is no need to add more antibodies or DNA–protein conjugates. In addition, the stable barcode DNA is removed from the detection assay, and direct detection is carried out on samples of barcode DNA that are free from most of the biological samples, MMPs, and AuNPs. This step substantially reduces background signal [176].

This protein detection scheme has the potential for massive multiplexing and the simultaneous detection of many analytes in one solution, especially in the PCR-less form [173, 177].

Table 8. Application of AuNPs based biobarcode assay as biomarker for disease detection.


Lateral flow immunoassay

AuNP-based dipstick assays or lateral flow immunoassays (LFIs) are the most commercially successful applications of AuNPs because they depend on their excellent optical properties and biocompatible [13, 22]. AuNPs also provide a stable surface for the immobilization of biomolecules with no loss of biological activity [188]. The membrane-based LFI strip test is a well-established and cost-effective technique that is appropriate for the rapid identification of various analytes in point-of-care situations. The LFI strip was first introduced to target human chorionic gonadotropin (hCG) for pregnancy detection [189]. Since its introduction in the late 1980s, the LFI strip has become a popular platform for the development of rapid tests because it eliminates the need for trained personnel and expensive equipment. The technique is widely used for the specific qualitative or semi-quantitative detection of many classes of analyte including antigens, antibodies, haptens, and even oligonucleotides. Common, commercially available hCG strips currently include those designed to test for pregnancy, and infections such as Streptococcus, Chlamydia, human immunodeficiency virus (HIV), and hepatitis C virus (HCV). It is increasingly applied to various fields such as clinical diagnosis, food safety testing, and environment monitoring [189, 190].

The LFI strip assay is based on the principles of immunochromatography and is presented in a yes-or-no format. A test strip typically consists of porous materials in four zones containing different reagents: a sample-loading pad, a conjugate pad containing AuNPs conjugated with antibody (Au-Ab), a detection membrane containing narrow absorbed bands of proteins as test and control lines, and an absorbent pad, as depicted in Figure 10. For the convenience of storage and handling, the porous materials are laminated with a semi-rigid material of appropriate mechanical strength. When a drop of sample (e.g., urine or blood) is applied to the sample pad, the liquid sample migrates by capillary diffusion through the conjugate pad and rehydrates the Au-Ab; the analytes then interact with the conjugate [191]. An immobilized capture antibody then binds to the complex and the accumulation of Au-Ab appears as a red line [90]. There are two lines on the membrane in the detection zone (test and control lines). The Au-Ab complex in the analyte then moves onto the membrane towards the capture target, where it becomes immobilized and concentrated, producing a distinct signal in the form of a sharp red line [189]. The function of the test line is to determine whether the sample is positive or negative. A second line, the control line, may also be formed on the membrane by trapping excess Au-Ab, indicating that the test is complete. The control line also functions as a validity test. The use of AuNPs in LFI to detect several diseases is expounded in Table 9.

Table 9. Selected applications of gold nanoparticle (AuNP)-conjugated biomolecules used as a detector in a lateral flow immunoassay (LFI) for disease detection.


Figure 10. Working principle of the lateral flow immunoassay (LFI) with the sandwich format applied to the detection zone.

There are two types of LFI assay: the sandwich and competitive formats. LFI based on the sandwich format is used to test for large analytes with multiple binding sites, such as viruses and antibodies [192]. When detecting antigens, target analytes in the sample effluent are recognized by the antibody-conjugated AuNPs, which form analyte–antibody complexes bound to the immobilized antibody on the test line; the excess conjugate is trapped by the antibody on the control line, forming two red lines on the membrane, which indicate a positive result, as shown in Figure 10. The line intensity is directly proportional to the amount of analyte present in the sample [193]. If only the control line appears, the result is negative. However, if only the test line appears, the test should be repeated owing to incorrect binding [22, 194].

The competitive format is usually used to detect small molecules with a single antigenic determinant such as DNA [195]. In the competitive format, free or unlabeled analytes in the sample block the binding sites of the antibodies and prevent the uptake of colored particles, as shown in Figure 11. Thus, the response is negatively correlated to the concentration of the analyte. When more analytes are present, a low signal is obtained, whereas a high signal is obtained in the absence of an analyte [193]. Consequently, only the control line appears in a positive result, whereas both the test and control lines appear in a negative result

Figure 11. Principle of lateral flow strip test using the competitive format.

Material selection is a critical criterion for the success of any lateral flow platform [196]. The membrane is the most important material in a lateral flow strip. The protein-binding capacity and capillary flow rate are the most critical parameters for membrane selection, and they are determined by the polymer composition, pore size, porosity, and thickness. The choice of a membrane can have a profound effect on the reliability of a rapid diagnostic test strip or device. Membrane manufacturers generally offer a wide variety of material types and pore sizes, and the optimum membrane for a specific use should be determined by trial investigations. In practice, nitrocellulose (NC) membranes with pore sizes of 5–15 μm supplied as supported or unsupported forms are the most commonly used [198]. The chart in Figure 12 illustrates which membranes allow the greatest particle mobility. It is important to test membrane mobility, as indicated by a transition from blue to white because these areas are variable. With AuNPs, there is little or no restriction on the flow of particles as they pass through the membrane. With larger beads, mobility may be restricted depending on the size of the particles and the viscosity of the sample [197]. The capillary flow rate is the speed at which a sample material moves along a membrane strip in a rapid diagnostic test strip or application. Moreover, there is a direct correlation between the flow rate and the sensitivity of the membrane. Here, the capillary flow rate is 1 s/4 cm. Therefore, for HF 240, HF 180, HF 135, HF 120, HF 90, and HF 75, the particles flow at 240 s/4 cm, 180 s/4 cm, 135 s/4 cm, 120 s/4 cm, 90 s/4 cm, and 75 s/4 cm, respectively.

Figure 12. Particle mobility versus particle diameter on Hi-Flow Plus membranes, with indications of capillary flow rate and sensitivity 

Conclusion

Gold nanoparticles (AuNPs) have been widely used in diagnostic applications. The size and shape of the AuNPs can be tuned by changing the synthesis parameters. AuNPs must be conjugated with biomolecules before they can be used in diagnostic applications. Therefore, a simple and efficient conjugation approach is needed. AuNPs have been used in electrochemical and optical biosensors. The choice of the type of biosensor used to detect a certain disease depends on the specificity, selectivity, and simplicity of the device.

Acknowledgement

The authors appreciate the technical support provided by the School of Materials and Mineral Resources Engineering and the Institute for Research in Molecular Medicine of USM. This research was funded by University Research grants 1001/PSKBP/8630019 and PGRS 1001/Pbahan/8034049.

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