Effect of organic vapors on Au, Ag, and Au-Ag alloy nanoparticle films with adsorbed 2,6-dimethylphenyl isocyanide.

The physicochemical properties of metallic substrates are affected by the environment in different ways. It is generally difficult to determine these effects because the molecules in the environment interact weakly with metallic substrates. In this work, we demonstrate that even the effect of volatile organic compounds (VOCs) can be identified by utilizing the surface-enhanced Raman scattering of isocyanide molecules. The NC stretching band of 2,6-dimethylphenyl isocyanide (2,6-DMPI) adsorbed on Au, for instance, is blueshifted by 6 cm(-1) under an acetone flow and is redshifted by 20 cm(-1) under an ammonia flow. The same band of 2,6-DMPI adsorbed on Ag and Au0.5Ag0.5 alloy films is, however, redshifted equally by 8 and 13 cm(-1) under acetone and ammonia flows, respectively. This indicates that although the surface plasmons of Au0.5Ag0.5 alloy nanoparticles are clearly distinct from those of Ag (as well as Au) nanoparticles, both Au0.5Ag0.5 and Ag nanoparticles show a similar response to VOCs. These observations led us to conclude that the outermost parts of Au-Ag alloy nanoparticles are enriched with Ag atoms and that only the surfaces of metal nanoparticles, and not the bulk material, are affected by VOCs.

Raman spectral characteristics of 4-aminobenzenethiol adsorbed on ZnO nanorod arrays.

Interest in the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) has surged recently. The SERS spectral features are highly dependent on the measurement conditions; a notable example is the appearance of b2-type bands that are not evident in the normal Raman (NR) spectrum. In an effort to discover new information and make any necessary corrections, we measured the Raman spectrum of 4-ABT adsorbed on a semiconducting material that would only enable the chemical enhancement mechanism; accordingly, the Raman spectrum of 4-ABT adsorbed on ZnO nanorods grown on an indium tin oxide substrate was measured for the first time. In the NR spectrum of the zinc salt of 4-ABT, which was taken as a reference, only the a1-type bands of 4-ABT were identified. However, in the surface Raman spectrum of 4-ABT on ZnO, the b2-type bands were also clearly evident, suggesting that the b2-type bands arose owing to its adsorption onto ZnO. The b2-type bands were also observed for 4-ABT analogs adsorbed on ZnO; this confirms that the b2-type bands were not a result of a surface-catalyzed photoreaction. Based on electric-potential and excitation-wavelength dependence studies, the a1- and b2-type bands were attributed to a charge-transfer (CT) transition from the surface defect levels of ZnO to the unoccupied La and Lb states (in Platt's notation) of 4-ABT, respectively; the bands gained intensity via the Herzberg-Teller coupling terms. The enhancement factor associated with the CT transition was estimated to be around 22, suggesting that it is, at best, a moderately effective process.

Cyanide SERS as a platform for detection of volatile organic compounds and hazardous transition metal ions.

It is demonstrated herein that cyanide adsorbed on nanostructured Au is a very useful system for the detection of not only volatile organic compounds (VOCs) but also transition metal ions by means of surface-enhanced Raman scattering (SERS). This technique exploits the susceptibility of the CN stretching frequency of cyanide on Au to the variation of the surface potential of the Au nanostructures that occurs in response to the adsorption of VOCs onto these surfaces. It also exploits the susceptibility to the binding of transition metal cations to the pendant nitrogen atom, which reduces the anti-bonding character of the nitrogen lone pair of electrons. The CN stretching band was illustrated to undergo a blue-shift by up to 20 cm(-1) in response to a typical biogenic VOC farnesol, whereas the band was red-shifted by 10 cm(-1) in response to the typical biogenic VOC (+)-α-pinene. This method is considered to be highly sensitive given that the peak shift of 2 cm(-1) could be reproducibly measured even at a partial pressure of 1 Pa, corresponding to 76 ppm of farnesol. The CN stretching band was also demonstrated to undergo a blue-shift by up to 60-64 cm(-1) in the presence of trivalent cations such as Fe(3+) and Cr(3+), whereas the band was blue-shifted by 26-35 cm(-1) in the presence of divalent metal ions such as Mn(2+) and Fe(2+). The present SERS method is regarded as very promising because transition metal ions were detectable at concentration levels as low as 1 fM.

Photoreduction of 4,4'-dimercaptoazobenzene on ag revealed by Raman scattering spectroscopy.

The surface-enhanced Raman scattering (SERS) of 4,4'-dimercaptoazobenzene (4,4'-DMAB) has recently seen a surge of interest, since it might be possible to form 4,4'-DMAB from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. We found in this study, however, that the reverse conversion of 4,4'-DMAB to 4-ABT on Ag is a more feasible process upon irradiation with a 514.5 nm (not 632.8 nm) laser under ambient conditions. First of all, the SERS spectral pattern of 4,4'-DMAB on Ag varied as a function of laser irradiation time, finally becoming the same as that of 4-ABT on Ag. Second, the coupling reaction with 4-cyanobenzoic acid to form amide bonds proceeded readily like 4-ABT once 4,4'-DMAB on Ag was exposed to 514.5 nm radiation. Third, the growth of a calcite crystal occurred on 4,4'-DMAB on Ag, also likely on 4-ABT, when it was exposed to 514.5 nm radiation beforehand. All of these results led us to conclude that the appearance of the so-called b(2)-type bands in the SERS of 4-ABT must be due to the involvement of the chemical enhancement mechanism, not due to the formation of 4,4'-DMAB.

Detection of a few of biogenic volatile organic compounds by means of Raman scattering of isocyanide-adsorbed gold nanostructures.

We demonstrate that the NC stretching band of 2,6-dimethylphenylisocyanide (2,6-DMPI) adsorbed on poly(ethylenimine)-capped Au film is very susceptible to the kind of biogenic volatile organic compounds (VOCs) exposed, suggesting that the isocyanide-adsorbed noble metal nanostructures can be used as a platform for a biogenic VOC sensor operating via surface-enhanced Raman scattering (SERS). Specifically, first we demonstrate that highly SERS-active Au films can easily be fabricated onto the inner surfaces of glass capillaries, being able to measure the SERS spectra of 2,6-DMPI facilely and thus to monitor the shift of its NC stretching band rapidly in response to a variety of biogenic VOCs including isoprene, farnesol, and (+)-α-pinene. Secondly, we are able to deduce from the NC stretching peak shifts that farnesol must act as an electron acceptor so as to increase the surface potential of Au nanoparticles, while isoprene and (+)-α-pinene are electron donors, resulting in the decrease in the surface potential of Au nanoparticles. To our knowledge, this is the first report, informing the applicability of SERS, though indirect, in the detection of biogenic VOCs.

Surface-enhanced Raman scattering characteristics of nanogaps formed by a flat Ag substrate and spherical Pt nanoparticles.

We estimated the apparent size of the 'hot site' for surface-enhanced Raman scattering (SERS) located within the gaps between Pt nanoparticles and a flat Ag substrate. Initially, no Raman peaks were detected for 4-aminobenzenethiol (4-ABT) on a flat Ag substrate. Upon attaching 68 nm-sized Pt nanoparticles onto the amine group of 4-ABT (thus denoted as Pt-4-ABT/Ag(flat)), Raman peaks were distinctly observed, not only with the excitation at 488 nm but also with the excitation at 632.8 nm. This means that electromagnetic 'hot site' had formed at the gaps between Pt nanoparticles and a flat Ag substrate. When 4-ABT molecules were adsorbed additionally onto the vacant sites of Pt nanoparticles in Pt-4-ABT/Ag(flat), the Raman signal did not increase further, suggesting that the SERS 'hot site' was very limited and located mostly at the gaps between Pt nanoparticles and a flat Ag substrate, in agreement with the finite-difference time-domain (FDTD) calculation. To a rough estimate, about 1000 molecules residing only within a ~15 nm diameter area of the center of the gap must have contributed most of the measured Raman signal of 4-ABT.

Raman scattering characterization of 1,4-phenylenediisocyanide in Au-Au and Ag-Au nanogaps.

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). In this sense, the characteristics of 1,4-phenylenediisocyanide (1,4-PDI) trapped in a nanogap formed by a flat Au or Ag substrate and 60 nm-sized Au or Ag nanoparticles have been examined by means of Raman scattering spectroscopy. It is noteworthy that the NC stretching band of 1,4-PDI is very susceptible to the measurement condition. The NC stretching band is observed at 2177, 2173, and 2174 cm(-1) when 1,4-PDI is trapped in the Au-Au, Ag-Au, and Au-Ag nanogaps, respectively, but the corresponding peak shifts linearly with a slope of as much as 22.4, 28.5, and 31.2 cm(-1)V(-1), respectively, in the electrochemical environment. On the other hand, the NC stretching peak is found to blue-shift by up to 8, 3, and 5 cm(-1), respectively, when the Au-Au, Ag-Au, and Au-Ag nanogaps are exposed to acetic acid. In contrast, in the presence of ammonia, the NC stretching peak is red-shifted by up to 9, 4, and 5 cm(-1), respectively. This can be understood by presuming that acetic acid acts as an electron acceptor, while ammonia acts as an electron donor when these volatile organics interact with Au or Ag, thereby resulting in either the increase or the decrease in the surface potential of the nanogap electrodes.

Polyethylenimine-capped Ag nanoparticle film as a platform for detecting charged dye molecules by surface-enhanced Raman scattering and metal-enhanced fluorescence.

Many drugs are charged molecules and are weak bases or acids having counterions. Their binding to biological surfaces is generally difficult to assess by vibrational spectroscopy. In this work, we demonstrated the potential of surface-enhanced Raman scattering (SERS) conducted using a polyethylenimine (PEI)-capped Ag nanoparticle film for the quantification of an electrostatic adsorption process of charged drug molecules, by using charged dye molecules such as sulforhodamine B (SRB) and rhodamine-123 (R123) as model drugs. It was possible to detect small-sized anions such as SCN(-) at 1 × 10(-9) M by SERS because of the cationic property of PEI. We were subsequently able to detect a prototype anionic dye molecule, SRB, by SERS at a subnanomolar concentration. On the other hand, it was difficult to detect cationic dyes such as R123 because of the electrostatically repulsive interaction with PEI. Nonetheless, we found that even R123 could be detected at subnanomolar concentrations by SERS by depositing an anionic polyelectrolyte such as poly(sodium 4-styrenesulfonate) (PSS) and poly(acrylic acid) (PAA) onto the PEI-capped Ag nanoparticles. Another noteworthy point is that a subnanomolar detection limit can also be achieved by carefully monitoring the fluorescence background in the measured SERS spectra. This was possible because charged dyes were not in contact with Ag but formed ion pairs with either PEI or PSS (PAA), allowing metal-enhanced fluorescence (MEF). The PEI-capped Ag nanoparticle film can thus serve as a useful indicator to detect charged drug molecules by SERS and MEF.

Selective detection of aqueous nitrite ions by surface-enhanced Raman scattering of 4-aminobenzenethiol on Au.

In this work, we have devised a selective nitrite-ion detection method based on the surface-enhanced Raman scattering (SERS) of 4-aminobenzenethiol (4-ABT) on Au. This is possible because, firstly, SERS is a very surface-sensitive technique with monolayer detection capability, and secondly, the amine group of 4-ABT reacts readily with nitrites in acidic media, forming a diazonium group, which can subsequently form an azo bond by reacting with a variety of benzene derivatives. From the peak intensity of the diazonium group, the presence of nitrite ions above 20 µM can be identified readily. From the peak intensity of the azo moiety alone, it is even possible to detect nitrite ions at concentrations as low as 5 µM, without interference from other anions. This work clearly illustrates the usefulness of SERS in environmental science research.

Organic isocyanide-adsorbed gold nanostructure: a SERS sensory device for indirect peak-shift detection of volatile organic compounds.

Organic isocyanide adsorbed on a noble metal nanostructure can be used as a platform for a volatile organic compound (VOC) sensor operating via surface-enhanced Raman scattering (SERS). This is possible since the NC stretching band of organic isocyanides such as 2,6-dimethylphenylisocyanide (2,6-DMPI) is very susceptible to the surface potential of Au onto which 2,6-DMPI is assembled. The surface potential of Au nanoparticles is even subject to change by VOCs, which can be easily monitored by the SERS of 2,6-DMPI. Thereby, under the flow of CCl(4) vapor at a partial pressure of 12.8 kPa, for instance, the NC stretching band is blue-shifted by up to 20 cm(-1) within 30 s, corresponding to a potential change of +0.56 V. Conversely, under the flow of butylamine at 12.8 kPa, the NC stretching band is red-shifted, instead of being blue-shifted, by as much as 12 cm(-1). At lower partial pressures, even a blue- or red-shift of 1 cm(-1) was reproducibly measured at a partial pressure of 125 mPa, corresponding to 6.5 ppm for CCl(4), suggesting that the present detection limit is superior to the results obtained via other techniques, especially those operating based on gold nanoparticles and aggregates.

Surface-enhanced Raman scattering of 4,4'-dimercaptoazobenzene trapped in Au nanogaps.

The surface-enhanced Raman scattering (SERS) of 4,4'-dimercaptoazobenzene (4,4'-DMAB), an alpha, omega-dithiol possessing also an azo moiety, has seen a surge of interest recently, since 4,4'-DMAB might be able to form from 4-aminobenzenethiol (4-ABT) via a surface-induced photoreaction. An understanding of the intrinsic SERS characteristics of 4,4'-DMAB is thus very important to evaluate the possibility of such a photoreaction. We found in this work that 4,4'-DMAB should adsorb on a flame-annealed Au substrate via one of its two thiol groups such that Au nanoparticles could adsorb further on the pendent thiol group, forming a SERS hot site. The most distinctive feature in the SERS of 4,4'-DMAB was the appearance of a(g) bands, which were quite similar to the b(2)-type bands occurring in the SERS of 4-ABT. In an electrochemical environment, the a(g) bands of 4,4'-DMAB at 1431, 1387, and 1138 cm(-1) became weakened at lower potentials, completely disappearing at -1.0 V, but the bands were restored upon increasing the electrode potential, implying that neither electro- nor photo-chemical reaction to break the azo group took place, in agreement with data from a cyclic voltammogram. The appearance and disappearance of these a(g) bands are thus concluded to be associated with the charge transfer phenomenon: 4,4'-DMAB must then be one of a unique group of compounds exhibiting chemical enhancement when subjected to a SERS environment.

Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength.

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.

Surface-enhanced Raman scattering: a powerful tool for chemical identification.

The nature of "hot spots" in surface-enhanced Raman scattering (SERS) and the novel fabrication of Ag thin films for efficient SERS measurements are the main focus of this review. By using 4-aminobenzenthiol or 1,4-phenylenediisocyanide molecules we can characterize the nanogap formed by a metal nanoparticle and a flat metal substrate, or the gap between two spherical metal nanoparticles. These nanogaps are indeed SERS-active sites and the apparent size of "hot spots" is found to be very limited. To use SERS for routine chemical identifications, the substrates should be stable, reproducibly prepared, inexpensive, and easy to make. We introduce a method for the facile fabrication of Ag thin films, simply by soaking glass substrates in ethanolic solutions of AgNO(3) and butylamine, and their application as efficient SERS substrates. Ag-coated glass capillary and Ag deposited magnetic nanoparticles can be used for the microanalysis of bio- and hazardous chemicals.

Effect of volatile organic chemicals on surface-enhanced Raman scattering of 4-aminobenzenethiol on Ag: comparison with the potential dependence.

4-Aminobenzenethiol (4-ABT) is an unusual molecule in the sense that several distinct peaks whose counterparts are rarely found in the normal Raman spectrum are observed in its surface-enhanced Raman scattering (SERS) spectra. Their origin has been argued over recently as due to either a metal-to-adsorbate charge transfer or the formation of a photoreaction product such as dimercaptoazobenzene (DMAB). In an electrochemical SERS measurement, the intensities of the new peaks depended strongly not only on the excitation wavelength but also on the electrode potential. Interestingly, we observed a similar spectral variation even under ambient conditions by exposure of 4-ABT on Ag to volatile organic chemicals (VOCs) such as acetone and ammonia. Since acetone and ammonia barely react directly with 4-ABT, the effect of VOCs must be indirect, presumably associated with the movement of electrons between VOCs and the Ag substrate causing either an increase or a decrease in the surface potential of Ag. Based on the potential-dependent SERS data, the effect of acetone therefore appeared to correspond to an application of +0.15 V to the Ag substrate vs. a saturated Ag/AgCl electrode, while the effect of ammonia corresponded to the application of -0.45 V to Ag. We admit that much the same VOC effect could be observable if a photoproduct was formed immediately upon irradiation and the product was also subjected to a chemical enhancement mechanism. The Gaussian response of the peak intensities of the b(2)-type bands to applied potential, as well as to VOCs, dictated that the new peaks appearing in the SERS of 4-ABT have nothing to do with any electrochemical reaction. In addition, a separate preliminary work suggested that the b(2)-type bands are not at least due to a photoreaction product such as DMAB.

Surface-enhanced Raman scattering of 4-aminobenzenethiol in Ag sol: relative intensity of a1- and b2-type bands invariant against aggregation of Ag nanoparticles.

4-Aminobenzenthiol (4-ABT) is an unusual molecule, showing variable surface-enhanced Raman scattering (SERS) spectra depending upon measurement conditions. In an effort to reduce ambiguity and add clarity, we have thus conducted an ultraviolet-visible (UV-vis) extinction measurement, along with Raman scattering measurement, after adding 4-ABT into aqueous Ag sol. Upon the addition of 4-ABT, the surface plasmon absorption band of Ag at 410 nm gradually diminished and, concomitantly, a weak and broad band developed at longer wavelengths, obviously because of the aggregation of Ag nanoparticles. At the same time, the Raman scattering peaks of 4-ABT varied in intensity as the Ag particles proceeded to form aggregates. A close examination revealed that the peak intensity of the ring 7a band of 4-ABT, a typical a(1) vibrational mode, could be correlated with the UV-vis extinction of the Ag sol measured at the excitation laser wavelength. In a separate Raman measurement conducted using sedimented Ag colloidal particles, 4-ABT was found not to be subjected to any surface-induced photoreaction, implying that all of the observable Raman peaks were, in fact, solely due to 4-ABT on Ag. The intensities of the b(2)-type bands, such as the ring 3, 9b, and 19b modes of 4-ABT, were then analyzed and found to be invariant with respect to the 7a band, irrespective of the extent of Ag aggregation as far as at a fixed excitation wavelength. The intensity ratio of the b(2)-type/7a bands would then reflect the extent of the chemical enhancement that was involved in the SERS of 4-ABT in aggregated Ag sol.

Effect of organic vapors and potential-dependent Raman scattering of 2,6-dimethylphenylisocyanide on platinum nanoaggregates.

The surface-enhanced Raman scattering characteristics of 2,6-dimethylphenylisocyanide (2,6-DMPI) on Pt nanoaggregates, in ambient and electrochemical environments and in the presence of organic vapors, were examined and compared with those on Au nanoaggregates. Due to the exclusive adsorption via the isocyanide group, the NC stretching band was very susceptible to the measurement conditions although the ring associated bands showed negligible peak shifts. In ambient conditions, the peak shift of the NC stretching vibration on Pt (29 cm(-1)) was one half of that on Au (61 cm(-1)), suggesting that the electron donation capability of the isocyanide group to Au was greater than that to Pt. In the electrochemical environment, the NC stretching peak varied linearly with slopes of ∼42 and ∼36 cm(-1) V(-1) on Pt and Au, respectively. On the other hand, the NC stretching bands of 2,6-DMPI on Pt red-shifted by as much as 15 and 41 cm(-1), in the presence of acetone and ammonia, respectively, corresponding to the lowering of the surface potential of Pt nanoaggregates from +0.2 to -0.2 and -0.8 V, respectively. On Au nanoaggregates, however, acetone appeared to increase the surface potential of Au from +0.2 to +0.3 V, although ammonia decreased the surface potential from +0.2 to -0.4 V. Acetone must then act as an electron donor when interacting with Pt while it serves as an electron acceptor when interacting with Au, in agreement with an ab initio quantum mechanical calculation.

Surface-enhanced Raman scattering of 4-aminobenzenethiol on gold: the concept of threshold energy in charge transfer enhancement.

Some threshold energy is required for 4-aminobenzenethiol to occupy the gold surface sites capable of leading to the appearance of surface-enhanced Raman scattering peaks via a charge transfer resonance enhancement mechanism.

In situ Raman monitoring of competitive adsorption of Ag and Au nanoparticles onto a poly(4-vinyl pyridine) surface.

The competitive adsorption of citrate-capped Ag and Au nanoparticles (~25 nm in diameter) onto a poly(4-vinyl pyridine) (P4VP) surface has been investigated by means of Raman scattering spectroscopy. The P4VP film prepared on a glass slide was too thin for its normal Raman spectrum to be observed, but the Raman peaks of P4VP could be detected upon the adsorption of Ag and/or Au nanoparticles onto the film, due to the surface-enhanced Raman scattering (SERS) effect associated with the localized surface plasmon of Ag and/or Au nanoparticles. Neither quartz crystal microbalance nor atomic force microscopy (AFM) nor scanning electron microscopy (SEM) methodologies can distinguish between Ag and Au nanoparticles during their adsorption onto P4VP, but it is possible through Raman scattering spectroscopy because Ag (though not Au) nanoaggregates are SERS active at 514.5 nm excitation, while both Ag and Au nanoaggregates are SERS active at 632.8 nm excitation. Coupled with the AFM data, we were thus able to infer that about 120 Ag nanoparticles per 1 µm(2) were adsorbed, along with 60 Au nanoparticles per 1 µm(2), onto the P4VP film over a period of 1.5 h from a 1 : 1 mixture of Ag and Au sols at 1.6 nM each.

Silver-coated dye-embedded silica beads: a core material of dual tagging sensors based on fluorescence and Raman scattering.

We have developed a new type of dual-tag sensor for immunoassays, operating via both fluorescence and surface-enhanced Raman scattering (SERS). A one-shot fluorescence image over the whole specimen allows us to save considerable time because any unnecessary time-consuming SERS measurements can be avoided from the signature of the fluorescence. Dye-embedded silica beads are prepared initially, and then SERS-active silver is coated onto them via a very simple electroless-plating method. The Raman markers are subsequently assembled onto the Ag-coated silica beads, after which they are stabilized by silanization via a biomimetic process in which a poly(allylamine hydrochloride) layer formed on the Raman markers by a layer-by-layer deposition method acting as a scaffold for guiding silicification. In the final stage, specific antibodies are attached to the silica surface in order to detect target antigens. The fluorescence signal of the embedded dye can be used as a fast readout system of molecular recognition, whereas the SERS signals are subsequently used as the signature of specific molecular interactions. In this way, the antibody-grafted particles were found to recognize antigens down to 1 × 10(-10) g mL(-1) solely by the SERS peaks of the Raman markers.

pH effect on surface potential of polyelectrolytes-capped gold nanoparticles probed by surface-enhanced Raman scattering.

Nanoparticles are commonly stabilized through the adsorption of acidic/basic polyelectrolytes around the surface of the particle. One example of these nanoparticles is poly(ethylenimine) (PEI)-capped Au nanoparticles. In this work, we have examined by means of surface-enhanced Raman scattering (SERS) of 2,6-dimethylphenylisocyanide (2,6-DMPI) how much the surface potential of Au nanoparticles is affected by the solution pH through the mediation of the protonation and deprotonation of PEI in contact with Au nanoparticles. In fact, the surface-potential-dependent isocyanide (NC) stretching peak of 2,6-DMPI has shifted sharply around pH 8.5, close to the pK(a) value of the primary amine of PEI. When a negatively charged poly(acrylic acid) (PAA) was deposited onto the PEI, the peak shift of the NC stretching band took place around pH 6.5, close to the average pK(a) value of PEI and PAA. When additional PEI was deposited on PAA, the peak shift of the NC stretching band occurred once again around pH 8.5, indicative of the stronger interaction of upper two polyelectrolyte layers. These data clearly illustrate the usefulness of SERS in the elucidation of a delicate interaction of cationic and anionic polyelectrolytes, especially in layer-by-layer deposition.