Silver nanomaterials for the detection of chemical and biological targets

Jae-Seung Lee

doi:10.1515/ntrev-2014-0017

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Silver nanomaterials for the detection of chemical and biological targets

Jae-Seung Lee
Jae-Seung Lee received his BS with summa cum laude in 2004 from the Korea Advanced Institute of Science and Technology (KAIST; Korea) and his PhD in 2008 from Northwestern University (USA). After his graduate training, he moved to the Massachusetts Institute of Technology (MIT, USA) as a postdoctoral associate. In 2009, he joined the faculty at Korea University (Seoul, Republic of Korea) as an Assistant Professor in Materials Science and Engineering, where he is now working as an Associate Professor. He is currently focusing on various interesting projects such as creating unique nanostructures by controlling the interactions of the building block nanomaterials, developing smart methods for modifying nanomaterial surfaces with various chemical and biological functionalities, and designing analytical strategies for detecting chemical and biological targets in the areas of medical, industrial, environmental, and military applications.

Veröffentlicht/Copyright: 16. September 2014

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Abstract

Silver nanomaterials have attracted a lot of interest from researchers, mainly owing to their distinctive optical properties such as localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS). These properties are particularly interesting for the diagnostic applications when combined with target-recognition capabilities of surface ligands and target concentration-dependent quantitative aspects of the LSPR or SERS signal. This review covers these two main optical properties of silver nanomaterials with respect to their sensing applications for various chemical and biological targets. The LSPR-based colorimetric detection schemes are further divided into two categories based on what they depend on: (1) assembly and (2) deformation of the silver nanoparticle probes. Because the various shapes of the silver nanomaterials are highly associated with how to design and control the LSPR- and SERS-based detection schemes, the latest progress in the development of highly sensitive and selective detection strategies are discussed with respect to the morphological diversity of the silver nanomaterials.

Keywords: assembly; detection; localized surface plasmon resonance; silver nanomaterials; surface-enhanced Raman scattering

1 Introduction

Recently, the improvement of methods to identify and recognize environmentally and biologically important targets has been emerging in the field of chemical sensors [1–5]. Especially DNA, proteins, metal ions, and small organic molecules are known to play important roles in a wide range of chemical and biological processes, and therefore, they have been of enormous interest to investigations aimed at conceptual development of sensing technologies for the past decades. Importantly, the visual diagnosis of those physiologically relevant targets with high sensitivity and selectivity would be essential to the elongation of expected life span and, thus, welfare of human beings. Conventional sensing materials, such as molecular chromophores, are limited to their low sensitivity and lack of ability to be functional for complicated sensing environment. Consequently, the colorimetric detection of chemical and biological targets based on nanomaterials, particularly silver ones, has become a highly attractive issue of interest to researchers working on the synthesis and applications of silver nanomaterials.

In order to successfully design the role and utility of silver nanomaterials in detection and diagnostics, it is essential to understand their chemical and physical properties. One of the unique optical properties of silver nanomaterials is their localized surface plasmon resonance (LSPR) [6–8]. LSPR is the collective oscillation of electrons (plasmons) in particulate solid materials (localized surface) stimulated by incident light (resonance) [9]. The resonance conditions are established when the light frequency is in accordance with the natural frequency of electrons in the solid materials oscillating against the restoring force of positively charged nuclei. When this phenomenon occurs in a flat surface, it is not localized anymore and is considered as general surface plasmon resonance (SPR). The comparison of SPR and LSPR is illustrated in Figure 1 [10]. Originally, surface plasmon, itself, was considered as a part of optics and has been known in such a way for more than 150 years [11]. Nowadays, however, the LSPR is considered to be strongly associated with nanotechnology because silver nanomaterials are active components to manipulate light and can be used as building blocks for more complicated structures to control light [12]. The intense and highly confined electromagnetic fields owing to the LSPR of the silver nanomaterials result in the very sensitive silver nanoprobes to identify and recognize even tiny changes in their dielectric surroundings. This novel optical property of silver nanoprobes is particularly attractive to diagnostic applications for various targets, especially compared to nanoprobes made of other plasmonic materials such as aluminum [13], nickel [14], copper [15–17], or conducting metal oxides [18], all of which have SPR wavelengths in the ultraviolet (UV) region. Gold is another frequently used plasmonic material whose plasmon absorption band is located in the visible range of the electromagnetic spectrum, along with silver. In terms of the signal intensity, however, gold nanoparticles exhibit lower extinction coefficients relative to silver nanoparticles of the same size and shape [19].

Figure 1

Schematic diagrams illustrating a surface plasmon (left) and a localized surface plasmon (right). The nanostructure is smaller than the wavelength of light, and the free electrons can be displaced from the lattice of positive ions (consisting of nuclei and core electrons) and collectively oscillate in resonance with the light. Reprinted with permission from Ref. 10. Copyright 2011 American Chemical Society.

The most popular and conventional detection strategy of these silver nanoprobes using their LSPR is based on the aggregation of the probes. When the silver nanoparticles aggregate, their electromagnetic fields are coupled, leading to the red shifts of their plasmonic bands depending on their interparticle distance [20, 21]. These red shifts induce a dramatic color change of the nanoprobes, which is a considerably valuable visual property as a signal for naked-eye recognition. Therefore, if multiple plasmonic nanoprobes are surface functionalized with target-binding ligands, they will bind to the targets owing to the attractive interactions and will aggregate into larger assemblies with a concomitant color change. Because the color of the dispersed silver nanoprobes before their aggregation can be efficiently determined in the entire visible range by controlling their sizes and shapes, ideally, one can design a detection scheme based on the visual change from any color to their red-shifted colors and, in certain cases, colorless.

Another emerging detection strategy based on the LSPR of the silver nanoprobes takes advantage of the deformation of the silver nanomaterials by hydrogen peroxide (H₂O₂), which is generated by enzymatic reaction of the target. Because the concentration of the enzymatically generated H₂O₂ is in proportion to the target concentration, the higher the target concentration is, the more deformation takes place. The shape-dependent LSPR changes sensitively in response to the deformation of the silver nanoplates by H₂O₂, especially at the end of the sharp tips, which first begin to be etched in the presence of H₂O₂. This assay scheme does not rely on the assembly of the nanoprobes but, on the other hand, does require their dispersed status in the reaction mixture for the accurate analysis of the target concentration without any other cause to the LSPR change. The high specificity based on the enzymatic reactions [22, 23], and the high sensitivity relying on the shape-dependent optical properties of silver nanomaterials [9, 24] are the key advantages of this detection strategy, which does not involve the assembly of the nanomaterials.

Enormous interest in understanding surface plasmon resonance spectroscopy has led to the discovery of surface-enhanced Raman scattering (SERS) [25, 26]. SERS is a surface-sensitive phenomenon that amplifies Raman scattering by particular molecules adsorbed on rough metal surfaces with nanostructures. The enhancement factor can be as high as 10¹⁰ to 10¹¹, indicating very high sensitivity of the SERS technique, enough to be utilized to detect even a single molecule. This unique phenomenon has been of interest to a number of researchers owing to its ambiguous mechanisms. Indeed, the exact mechanism of the enhancement effect of SERS is still an important issue of debate in the SERS community. Two primary competing mechanisms have been proposed to explain the observed Raman enhancement on roughened metallic substrates: (1) chemical enhancement and (2) electromagnetic field enhancement [27, 28]. These two mechanisms are different from each other significantly, but it is still unclear to distinguish them using experimental methods. In particular, the electromagnetic field enhancement mechanism is believed to be associated with surface plasmons because it requires coupling of the incident irradiation to the metal surface. Therefore, theoretical and experimental investigation of surface plasmons is essential to understand and manipulate SERS. A variety of silver nanomaterials with different sizes, shapes, and local dielectric properties have been studied for SERS because those experimental parameters of the silver nanomaterials would determine the LSPR wavelengths. Importantly, these studies have allowed one to fundamentally understand how surface plasmons of silver nanomaterials are affected by their local nanostructures and environments. Moreover, silver nanomaterials have emerged as promising and useful diagnostic tools with high sensitivity and selectivity owing to their sensing applications of the SERS properties.

In this review, a series of recent reports describing detection schemes for biological targets such as protein, DNA molecules, and chemical targets including small organic reagents using a variety of silver nanomaterials will be discussed. Importantly, two distinctive, mainstream strategies based on the LSPR (assembly formation and deformation) will be introduced with representative examples first. Subsequently, several advanced examples of SERS-based detection will be described in detail. In each section, synthesis of silver nanomaterials will be briefly introduced with respect to their shapes and corresponding optical properties [29].

2 Detection strategies using various silver nanomaterials and their optical properties

2.1 LSPR-based detection using the assembly formation of silver nanomaterials

Assembly of silver nanoparticles is the key property of high-sensitivity detection schemes using LSPR [9, 30–35]. These assays are capable of visualizing the colorimetric signals, which often are analyzed using UV-vis spectroscopy and even can be recognized by the naked eye. During the development of colorimetric assays using silver nanomaterial assemblies, the color change that is generated during the assembly formation of silver nanoparticles was employed. The silver nanoparticles, in such cases, play an important role as nanoprobes to “sense” interactions between the target-recognition functionalities on the particle surface and the target. The most popular results based on such a nanoparticle assembly-based scheme have been comprehensively studied for the detection of DNA targets. For example, nanoparticle assemblies can be induced by combining two different types of DNA-modified silver nanoparticle (DNA-AgNP) probes with the target DNA sequence [36]. The two probe DNA sequences on the DNA-AgNPs are not complementary to the other, and therefore, the two types of DNA-AgNPs do not interact with each other and do stay dispersed in the absence of the target DNA. The two sequences, however, are designed to be complementary to each half of the target DNA sequence, respectively, and are supposed to lead to the sandwich-structured hybridization of the DNA strands when combined with the target sequence (Figure 2). This scheme is first reported as a strategy to reversibly assembly nanoparticles but, nowadays, is widely used with different types of nanoprobes for the colorimetric detection of DNA targets as a fundamental DNA-hybridization design. Importantly, such assembly formation is accompanied by distinctive color changes. The disassembly of such assembly DNA-AgNPs could be induced by decreasing the salt concentration of the mixture solution [37] or increasing the temperature above the melting temperature of the participating DNA sequences [38]. Furthermore, the melting temperature or the salt concentration at which the dehybridization takes place is known to be proportional to the concentration of the targets, indicating that this type of the assembly-based detection system could be a quantitative measure to analyze the concentration and amount of the unknown samples [38]. Importantly, such color changes of the reaction mixtures based on the presence of the target sequence can even allow one to clearly distinguish perfectly matched and a single base-mismatched sequences. Typically, the limit of detection (LOD) of detection methods based on the nanoparticle assemblies is several nanomolar or hundreds of femtomolar levels.

Figure 2

(Top) The gradual spectral change during the hybridization process of DNA-silver nanoprobe conjugates and the target DNA sequence. (Bottom) The colorimetric response can be recorded at various temperatures under controlled conditions. Reprinted with permission from Ref. [36]. Copyright 2008 American Chemical Society.

2.1.1 Silver nanospheres (AgNSs)

The LSPR-based detection methods using assembly of spherical silver nanoparticles, or silver nanospheres (AgNSs), were first reported in 2008 by Graham et al. [36]. Prior to their work, the AgNSs were demonstrated to be stably modified with probe DNA strands containing triple cyclic disulfide moieties by Mirkin et al. [39, 40]. Before this work, a number of assembly-based colorimetric detection schemes have been developed for the detection of DNA targets, but they were mainly with gold nanoparticles (AuNPs), which are the prototype of the DNA-nanoparticle conjugates. While reliable and versatile, however, these AuNPs were limited to a single-type color, red, owing to the limited range of the wavelength where the maximum absorbance takes place (∼530 nm). This limited availability of the color is the largest hurdle to enlarge the diversity of the visual signal types, which should be overcome for multiplexed assay schemes designed for multiple targets. In addition to gold, silver was strongly proposed as an alternative material owing to its intense plasmonic properties when its size is confined in several nanometers. The surface modification of AgNSs with probe DNA sequences, however, was not easily achieved by the single thiol-anchoring group, which was widely used for the DNA conjugation of AuNPs, owing to their instability after the conjugation procedure. Very interestingly, although sulfur-silver interactions were known to be the primary motivating force to form self-assembled monolayers (SAMs) of thiol ligands on a flat silver surface [41], they were not effective for the spherical silver nanoparticles and led to failures, except a few successful cases [42–44].

The utilization of cyclic disulfide anchoring group dramatically increased the stability of the DNA-AgNSs, possibly owing to the stronger bond formation between the AgNS surface and sulfur atoms in the anchoring group of the probe DNA. In fact, the stability of the DNA-AgNS conjugates in the presence of Na⁺ and Cl^- (up to 1 m of both ions) was as high as that of the normal DNA-AuNP conjugates. The DNA-AgNSs also exhibited the sharp melting transitions and salt concentration-dependent melting temperatures, which are typically the key properties of the DNA-nanoparticle conjugates [38]. Almost simultaneously and independently, a single-moiety cyclic disulfide anchoring group was also reported to be effective enough to functionalize DNA strands on AgNSs [45]. The introduction of the cyclic disulfide-anchoring groups for the DNA functionalization of AgNSs is very important because it opened up a new avenue in the investigation of the first non-gold plasmonic nanoparticles as nanoprobes for the detection of DNA targets.

2.1.2 Silver nanoplates (AgNPLs)

Unlike spherical nanoparticles, plate-like nanoparticles, in general, exhibit distinctive features owing to (1) their atomically crystalline structures, (2) lack of surface curvature, and (3) specific surface ligands [46–53]. These differences were huge obstacles for the functionalization of such plate-like nanoparticles with thiol DNA strands because of the unfavorable interactions between the particle surface and thiol DNA. Although a number of synthetic schemes were developed and reported for silver nanoplates (AgNPLs) with controlled sizes and shapes, their DNA conjugation had not been reported until Lee et al. published their work in 2010 [54]. Two main reasons can be explained for the success of the DNA-AgNPL conjugation. (1) In their work, the AgNPLs were synthesized by a seed-mediated method using polymers as a structure-directing reagent at the seed-synthesis step, not at the growth step [55]. Therefore, the surface of the AgNPLs was covered only with a typical weak ligand, citrate anion, which can be easily replaced with thiol DNA strands. (2) Furthermore, when combined with the AgNPLs, the concentration of DNA was three times higher (3 OD/ml) than the typical thiol DNA concentration (1 OD/ml) employed for gold nanoparticles [56], shifting the chemical equilibrium of the conjugation to the right based on the Le Chatelier’s principle. Interestingly, the plasmonic absorption band of the AgNPLs slightly blue shifted after the DNA conjugation, owing to the etching of the sharp tips by thiol DNA during the conjugation [54].

Importantly, these DNA-AgNPL probes exhibited excellent cooperative dehybridization properties, indicative of the dense DNA coverage on the AgNPL surfaces, as observed with gold nanoplates [57]. The melting temperature of the DNA-AgNPL probes increased as the size of the AgNPLs increased, owing to the increased number of the DNA duplex interconnects between the AgNPLs. This phenomenon was previously observed with spherical gold nanoparticles [58]. Moreover, the melting temperature also proportionally increased as a function of the target DNA concentration, an important quantitative and analytical feature of the detection system based on the AgNPL probes. Most of all, the highlight of this work is the demonstration of DNA-AgNPL probes whose color ranges in the entire rainbow spectra from red to violet (Figure 3). These DNA-AgNPLs reversibly assembled in the presence of target DNA strands with distinctive color changes (from each color to colorless), but disassembled back to their original colors when heated. This result illustrates the versatility of the AgNPLs as a colorimetric nanoprobe, particularly based on their structure-dependent capability to exhibit the full range of colors in the visible range, which can be easily achieved by controlling their sizes.

Figure 3

(A) A scheme depicting the design of the colorimetric detection system. (B) Melting transitions of DNA-AgNPR aggregates with respect to various target concentrations. The melting temperature (T_m) is plotted as a function of the target concentration (inset). (C) Melting transitions of DNA-AgNPR aggregates formed with a perfectly matched target (black line) and a single-based mismatch target (red line). The first derivatives of melting transitions are shown in the inset. (D) Rainbow colors exhibited by two complementary DNA-AgNPRs of various sizes before hybridization (top), after hybridization (center), and after melting (bottom). Reprinted with permission from Ref. [54]. Copyright 2010 American Chemical Society.

2.1.3 Silver nanocubes (AgNCs)

Silver nanocubes (AgNCs) are typically synthesized using PVP as a structure-directing reagent. PVP has been known as one of the most effective reagent for the shape control of the silver nanomaterials, which has been very intensively studied for the synthesis of AgNCs by Xia et al. [9, 59–65]. The optical properties of the AgNCs are not far different from those of AgNSs owing to their isotropic structures, which results in a single absorption plasmonic band at ∼400 nm. Unlike the AgNSs, however, their (1) non-curved flat surfaces and (2) single crystallinity are particularly attractive for controlling their chemical and physical properties suitable as nanoprobes for various detection schemes. The surface functionalization of AgNCs with thiol DNA was a fascinating challenge for their applications to the colorimetric detection of DNA targets. Because of the surface-remaining PVP molecules that cover the AgNC surfaces and block the contact between the surface and the thiol DNA, however, the surface modification of the AgNCs was somewhat problematic. In fact, although the first PVP-based synthesis of AgNCs was first reported by Xia et al. in 2002 [61], they were, for the first time, demonstrated as LSPR-based nanoprobes for the colorimetric detection of DNA after 10 years in 2012 [66].

For the efficient DNA functionalization of the AgNCs, the PVP “cover” layer was removed out of the AuNC surfaces by repeated washing with acetone. After the acetone washing, the AgNCs were fully functionalized with thiol DNA and exhibited unique melting profiles depending upon the spacer length of the probe DNA, salt concentration in the medium, and target DNA concentration. The loading of DNA strands also proportionally increased as a function of the salt concentration during the conjugation. As observed with other types of DNA-silver nanoparticle conjugates [54, 36], they exhibited distinctive color changes from yellow to pale pink in the presence of the target DNA, indicative of the target-recognition abilities of the DNA-AgNC probes (Figure 4). Very interestingly, these probes demonstrated curvature-dependent melting profiles, which was clearly different from the previous observation of other spherical nanoparticles. Considering the complicated nature of nanoparticle assemblies and their corresponding optical properties, the demonstration of such controllable assembly properties based on the surface curvature would be potentially useful for further complicated detection schemes [67–69].

Figure 4

(A) A series of melting transitions at different target sequence concentrations from 1 to 100 nm. (B) A plot of the melting temperature as a function of the target concentration. The color changes of the DNA-AgNCs before hybridization, after hybridization, and after melting are demonstrated in the inset. Reprinted with permission from Ref. [66]. Copyright 2012 American Chemical Society.

2.1.4 Silver nanowires (AgNWs)

Silver nanowires (AgNWs) whose aspect ratios are even higher than those of shorter silver nanorods [70, 71] are attractive as one-dimensional structures because they exhibit excellent optical, thermal, and electrical properties, which are typically suitable for conductive films, plasmon resonators, electrodes, and building blocks for other nanostructures ([72–75], [62]) Although their diameters are typically supposed to be smaller than 100 nm (which is why they are called “nano”wires), their length ranges over a few to tens of micrometers. Owing to their micrometer-scaled length in one dimension, the LSPR of AgNWs at a specific wavelength is hardly expected to be observed, especially when they are orientated randomly in a solution phase. Meanwhile, for analytical and quantitative applications of those AgNWs, the narrow length distribution of AgNWs is also important, which, however, has not been achieved successfully until recent years [76–78]. A new chemical method for AgNWs using poly(sodium 4-styrene sulfonate) (PSSS) has enabled the synthesis of water-dispersed AgNWs (50 nm in diameter) with controllable lengths from 0.5 to 2.5 μm [79]. These AgNWs can be surface-functionalized with thiol DNA sequences that can “capture” the target DNA sequence of which the other half is designed to hybridize with the DNA-gold nanoparticle probes (Figure 5). Either the decreased absorbance or the increased number of the DNA-gold nanoparticle probes can be quantitatively analyzed by UV-vis spectroscopy or electron microscopy, respectively (Figure 5). This method is straightforward, while highly sensitive to detect down to 50 pm of the target DNA sequence.

Figure 5

(A) Scheme depicting the detection of DNA targets using DNA-AgNW probes and DNA-AuNP probes. (B) SEM images of DNA-AgNW probes hybridized with DNA-AuNP probes and target DNA of various concentrations. (C) A graph showing the number of DNA-AuNPs per DNA-AgNW as a function of the target DNA concentration. (D) UV-vis spectra of DNA-AuNP probes dehybridized from the DNA-AgNW probes. (E) A graph showing the extinction of the spectra in (D) at 525 nm as a function of the target DNA concentration. Reprinted with permission from Ref. [79]. Copyright 2012 American Chemical Society.

2.1.5 Hierarchically branched silver nanostructures (HBAgNSs)

The large nanostructures, which have a domain size ranging around several micrometers, but has local nanostructured morphologies, were synthesized using various methods, such as electrochemistry, mixed solvents, mixed surfactants, ultrasonication, solid-based replacement reactions, microwave irradiation, and small organic molecules [80–92]. These structures, however, were mainly investigated with respect to their synthetic mechanisms and physical properties and have rarely demonstrated their useful applications. Recently, Pluronic triblock copolymers were used to synthesize hierarchically branched silver nanostructures (HBAgNSs) in an aqueous solution phase [93]. These structures exhibit locally three-dimensional hierarchy consisting of relatively long first-generation branches and shorter second and third generations, providing large surface area. The types of Pluronic polymers and their concentrations were critical parameters to determine their overall morphologies and sizes.

Although HBAgNSs do not exhibit noticeable optical properties in the visible range, they can still play a role as a plasmon quencher. Because of their metallic nature and microsize domains, the plasmonic nanoparticles adsorbed on the HBAgNS surface cannot exhibit the LSPR, thus, losing their absorption properties and corresponding colors. Based on this phenomenon, multiplexed DNA detection scheme was designed, where the presence of specific target DNA sequences could be identified by the color change of the reaction mixtures (Figure 6). Specifically, three different nanoprobes that exhibit either red (gold nanospheres), yellow (AgNSs), or blue (AgNPLs), respectively, were functionalized with capture DNA strands for the half of the three different target DNA sequences, respectively, and combined with HBAgNSs whose surface was also functionalized with the other three capture strands for the other half of the target sequences. In the presence of a specific target sequence, the corresponding nanoparticle probe is supposed to hybridize with the HBAgNSs and lose its color owing to the distance-distant optical properties of plasmonic nanoprobes based on the near-field electromagnetic coupling. Importantly, the presence and absence of the three colors based on the hybridization of each type of nanoprobes result in one of the possible eight colors (red, yellow, blue, green, orange, violet, black, colorless) determined by their eight combinations. This work is a meaningful demonstration of using HBAgNSs as an ideal platform to regulate the LSPR of other plasmonic nanomaterials, based on the large surface area from the nanostructured local branches, and mechanical stability originating from their microstructured domains.

Figure 6

(A) A scheme depicting the multiplexed colorimetric detection of three DNA target sequences (A, B, and C). Each target is designed to hybridize to the corresponding nanoparticle probe sequence and the HBAgNSs. (B) The HBAgNSs combined with three types of DNA-nanoparticle probes and target combinations before their hybridization, and (C) after hybridization. (D) The nanoparticle probe mixtures without HBAgNSs and targets, whose colors are in good agreement with those of the mixtures in (C), are shown as a color reference. Reproduced from Ref. [93] with permission from The Royal Society of Chemistry.

2.2 LSPR-based detection using the deformation of silver nanomaterials

The sensitive LSPR response of the silver nanomaterials has been recently demonstrated with mainly AgNPLs [94–96]. This is because of the structural advantages of the AgNPLs with the sharp edges and tips, which are easily etched by the reaction with corrosive H₂O₂. During the etching, the AgNPLs go through both the size reduction and shape deformation into smaller discoidal structures, resulting in the dramatic color change from blue to pink. These optical properties are highly attractive when combined with enzymatic reaction of targets to generate H₂O₂, resulting in both high sensitivity and selectivity. Because the assembly-based colorimetric detection schemes could inevitably generate false signals owing to the unexpected nonspecific attractive interactions between the nanoprobes, this “deformation-based” scheme would be suitable for enhancing the selectivity of the assay. In 2013, glucose was first detected using the oxidative etching of the AgNPLs, where glucose was enzymatically oxidized by glucose oxidase, leading to the generation of H₂O₂ [95]. The assay can selectively recognize glucose out of other physiologically relevant chemicals including amino acids and other glucose analogs (Figure 7). A similar, but different, assay scheme was later published in 2014, where DNA target sequences played a role as an initiator DNA in the hybridization chain reaction (HCR) (Figure 8) [94]. In the HCR, a linear DNA duplex with repeated units containing biotin molecules is generated by the programmed assembly of the designed sequences. The avidin-glucose oxidase conjugates are added to the HCR-generated duplex and bound to the biotin sites, resulting in the generation of H₂O₂ in the presence of glucose. Because the length of the duplex DNA is proportional to the concentration of the target DNA, the generated H₂O₂ would be also proportional to the target concentration. This work takes advantage of the amplification based on the combination of the HCR and structure-dependent LSPR, which is the fundamental cause of the high sensitivity of the assay (6 fm). The sequence-specific DNA-DNA duplex formation allows this detection system to distinguish even a single-base mismatch. Recently, these schemes have been modified in an inverse way, where the target molecules (uric acid) “protect” the specific facet such as (110) of the AgNPLs from being etched by excess H₂O₂ [96]. Therefore, the plasmonic shift is supposed to be reciprocally proportional to the concentration of the target (Figure 9). This scheme also demonstrates a very high sensitivity (LOD=10 nm) and selectivity, showing a great promise of using AgNPLs for other diagnostic applications.

Figure 7

(A) SPR peak shift (Δλ) of the (AgNPL)-GOx incubating with 100 μm glucose in the presence of various potential interfering substances. (B) The normalized SPR absorption spectra of (AgNPL)-GOx after 40 min of incubation with glucose (50 μm), fructose (500 μm), lactose (500 μm), and maltose (500 μm), respectively. Reprinted with permission from Ref. [95]. Copyright 2013 American Chemical Society.

Figure 8

(A) DNA detection scheme using the HCR. (B) The sensing mechanism is based on the etching process of triangular AgNPLs. Reprinted with permission from Ref. [94]. Copyright 2014 American Chemical Society.

Figure 9

(A) Colorimetric detection of uric acid using (1) the face-specific binding of uric acid on the AgNPLs, and (2) the etching of the AgNPLs using excess H₂O₂. (B) SPR peak shift (Δλ) as a function of concentrations of added uric acid. The inset exhibits a linear relationship between Δλ and c^1/3. Reproduced from Ref. [96] with permission from Elsevier.

In spite of the aforementioned advantages, however, this deformation method should suffer from a few, but obviously substantial, drawbacks. Once deformed, the silver nanomaterials are permanently damaged and cannot be restored back to their original shapes with the corresponding original optical properties. Compared to the reversible assembly-based detection schemes where the silver nanoparticle probes are reusable, this “irreversible” scheme is supposed to be costlier, and also environmentally less benign, considering the wasted silver ions. Another issue would be the selectivity of H₂O₂ to silver nanomaterials. That is, not only H₂O₂ but also other reactive chemical species also can etch the silver nanomaterials. For example, the concentration of cellular glutathione is as high as 5 mm, which is high enough to etch the AgNPLs [97]. Moreover, if H₂O₂ already exists in the detection environment such as cells, the detection system does not exhibit the quantitative H₂O₂ dependence anymore [98]. Additional procedures to take care of these potential problems prior to the detection using the deformation of AgNPLs might be necessary for their selective etching by H₂O₂, but further intense investigation would be required to fundamentally address these issues.

2.3 SERS-based detection using the silver nanomaterials

2.3.1 Silver nanocubes (AgNCs)

Silver nanoparticles containing sharp tips have been broadly used for various applications based on SERS. This is because the sharp edges of silver nanoparticles are found to affect SERS quantitatively, which has been proved experimentally. For example, one can compare the enhancement factors of (1) sharp tips and (2) truncated corners of AgNCs for detecting adsorbed molecules on the AgNC surfaces. Under the laser irradiation at 514 nm for excitation, the LSPR of the AgNCs, whether with sharp tips or with truncated corners, is supposed to be largely overlapped with the laser. Eventually, the truncated AgNCs exhibit an enhancement factor of 7.45×10⁴. In case of the AgNCs containing sharp tips, however, the enhancement factor increased nearly two times to 1.26×10⁵, indicating the importance of the sharp tips for the SERS signal amplification.

2.3.2 Silver nanospheres (AgNSs)

AgNSs are generally known as one of the “least efficient” nanoprobes for SERS because of the absence of their sharpness. Instead, the controlled assembly of AgNSs has been intensively investigated as an alternative method to provide the sufficient enhancement effect. The most attractive and interesting phenomenon in this regard is the creation and utilization of a “hot spot”, which is a gap area between a pair of strongly coupled gold or silver nanomaterials. These hot spots can amplify the electromagnetic fields in the gaps dramatically, even enough to be applied for the single molecule detection. In order to generate the hot spots for SERS amplification, AgNSs were dimerized in a solution phase by the polyol synthetic procedure in the presence of sodium chloride [99]. Importantly, the gap between the AgNSs is about 1.8 nm, small enough to exhibit the hot spot amplification, while large enough for target molecules to be trapped in the gap region. As a proof-of-concept, SERS spectra of 4-methylbenzenethiol (4-MBT) located in the hot spot were obtained repeatedly at various laser polarizations, whose maximum enhancement factor was calculated to be 1.9×10⁷. Although this work is limited to the detection of a specific Raman-active dye molecule as a target, it has demonstrated a general platform of a hot spot using AgNSs for the drastic enhancement of the SERS signal and is expected to be extended to the diagnostic applications for other targets after the Raman-dye labeling. Moreover, such dimer-like and trimer-like clusters of AgNSs in polymer shells were demonstrated as hot-spot platforms with high enhancement factors using different Raman dyes (2-naphthalenethiol) [100].

2.3.3 Silver nanowires

Single AgNWs were demonstrated to exhibit SERS enhancement properties in combination with metallic substrates such as thin films whose thickness is around 10∼200 nm. Interestingly, the SPR of the metallic thin films and LSPR of the AgNWs can be coupled strongly and affect the SERS properties beneficially. For example, the dielectric substrate such as silicon, when coupled with single AgNWs, can enhance the SERS signal only one-tenth 10 times of those with metallic substrate [101]. In comparison to gold nanowires, the AgNWs demonstrated improved SERS enhancement effects by a factor of 2. In the absence of the substrates, the AgNWs still exhibit the SERS enhancement by assembling in Langmuir-Blodgett monolayers in an ordered way on a water surface [102]. Because of the well-defined nanostructures of the AgNW layers, several Raman active dyes were evaluated for determining the enhancement factors and limit of the detection. Although the enhancement factors differed depending on the types of dyes, it was estimated to be 2×10⁹ in case of rhodamine-6G, truly efficient and promising for various applications. Later, vertically bundled AgNWs were synthesized using porous aluminum oxide templates and also determined to be excellent SERS substrates for the signal enhancement (25-fold) with benzenethiol [103]. These results obtained with assembled AgNWs illustrate the importance of hot spots for the signal amplification of SERS.

3 Conclusions

The primary purpose of this review is to serve as a milestone to summarize the recent various analytical schemes using the (1) LSPR and (2) SERS properties of silver nanomaterials with various shapes and sizes. Much work has been conducted to date on the development of LSPR- and SERS-based sensing systems. These assays have exhibited greater excellence over the conventional organic chromophore- or fluorophore-based sensing systems. While efficient and highly selective and sensitive under controlled conditions, however, these detection schemes still need to be improved in terms of the robust and practical applications for monitoring water quality, investigating forensic evidence, sensing chemical and biological warfare agents, and setting things up for point-of-care detection [104–110], all of which remain as a challenge. Moreover, the silver nanomaterials must be synthesized in a way that ensures accurate structural properties such as the size and shape. These nanomaterials also should be highly stable against aggregation. The surface functionalization of the silver nanomaterials with target-recognition ligands also needs to be carefully controlled in terms of their coverage on the silver nanomaterial surface, to avoid any possible loss of their reactivity. The development of any reliable techniques to recycle or reuse the used silver nanomaterial probes is an important issue for economic and environmental points of views.

In spite of the aforementioned remaining hurdles to be solved, however, the sensing schemes based on LSPR and SERS of silver nanomaterials still offer a number of promising properties and potentials. All those features need to be collected together to design highly selective and sensitive detection systems that might be comparable to the conventional sensing materials. Evidently, reliable synthetic recipes for complex silver nanomaterials will support the development of LSPR-based and SERS-based detection schemes. In this regard, the combination of silver nanomaterials with various types of other “smart” nanomaterials such as up-conversion nanoparticles and two-dimensional materials would be required for the enhanced signal amplification in the near future. The utilization of biological and bio-inspired materials for target recognition will also increase the selectivity by reducing the level of noise [111].

Corresponding author: Jae-Seung Lee, Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea, e-mail: jslee79@korea.ac.kr

About the author

Jae-Seung Lee

Jae-Seung Lee received his BS with summa cum laude in 2004 from the Korea Advanced Institute of Science and Technology (KAIST; Korea) and his PhD in 2008 from Northwestern University (USA). After his graduate training, he moved to the Massachusetts Institute of Technology (MIT, USA) as a postdoctoral associate. In 2009, he joined the faculty at Korea University (Seoul, Republic of Korea) as an Assistant Professor in Materials Science and Engineering, where he is now working as an Associate Professor. He is currently focusing on various interesting projects such as creating unique nanostructures by controlling the interactions of the building block nanomaterials, developing smart methods for modifying nanomaterial surfaces with various chemical and biological functionalities, and designing analytical strategies for detecting chemical and biological targets in the areas of medical, industrial, environmental, and military applications.

Acknowledgments

This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (Grant No.2012R1A1A2A10042814), and a Korea University Grant.

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Received: 2014-7-1

Accepted: 2014-8-13

Published Online: 2014-9-16

Published in Print: 2014-10-1

Artikel in diesem Heft

https://doi.org/10.1515/ntrev-2014-0017

Schlagwörter für diesen Artikel

assembly; detection; localized surface plasmon resonance; silver nanomaterials; surface-enhanced Raman scattering