A SERS-based pH sensor utilizing 3-amino-5-mercapto-1,2,4-triazole functionalized Ag nanoparticles.

We report the first use of 3-amino-5-mercapto-1,2,4-triazole (AMT) to construct a surface-enhanced Raman scattering (SERS) based pH nano- and microsensor, utilizing silver nanoparticles. We optimize the procedure of homogenous attachment of colloidal silver to micrometer-sized silica beads via an aminosilane linker. Such micro-carriers are potential optically trappable SERS microprobes. It is demonstrated that the SERS spectrum of AMT is strongly dependent on the pH of the surroundings, as the transformation between two different adsorption modes, upright (A form) and lying flat (B form) orientation, is provoked by pH variation. The possibility of tuning the nanosensor working range by changing the concentration of AMT in the surrounding solution is demonstrated. A strong correlation between the pH response of the nanosensor and the AMT concentration in solution is found to be controlled by the interactions between the surface and solution molecules. In the absence of the AMT monomer, the performance of both the nano- and microsensor is shifted substantially to the strongly acidic pH range, from 1.5 to 2.5 and from 1.0 to 2.0, respectively, which is quite unique even for SERS-based sensors.

Mellitic Acid-Supported Synthesis of Anisotropic Nanoparticles Used as SERS Substrate.

A method for the synthesis of a new SERS substrate-anisotropic silver nanoparticles using mellitic acid as a new capping agent is presented. The synthesis is free of toxic substances and does not require special temperature or lighting conditions. Moreover, it is fast, easy, and inexpensive. Depending on the concentration of silver ions and nanoparticle seeds, four different colloids were obtained, representing the evolution of nanoparticle growth along different paths from the first common stage. One of the synthesized colloids consists mainly of triangular nanoplates, while the other consists of polyhedral NPs. The analysis of the synthesis process together with the observation of TEM images and UV-vis extinction spectra enabled the proposal of the mechanism of interaction of mellitic acid molecules as the capping agent. The ability of mellitic acid molecules to form a hydrogen bond network, together with a ratio of silver ions to the mellitic acid concentration, turned out to be crucial for determining the shape of the NPs. All obtained colloids strongly enhance the Raman spectra of analyte molecules, thus proving their applicability as efficient new SERS substrates. For the one that enhanced the spectra the most, the detection limit was set at 10-9 M. Using it as a SERS substrate enables the identification of a trace amount of a designer drug, i.e., 4-chloromethcathinone (4-CMC, clephedrone). For the first time, SERS spectra of this substance, illegal in many countries, are presented.

Adsorption of enantiomeric and racemic cysteine on a silver electrode--SERS sensitivity to chirality of adsorbed molecules.

The adsorption of both (L- and D-) enantiomeric forms of cysteine on the silver electrode surface was studied by surface-enhanced Raman scattering spectroscopy (SERS) as a function of electrode potential and pH value of the solution. It was demonstrated that at potentials more positive than -0.7 V (for pH 3) or -0.8 V (for pH 2 or lower), in acidic environment L-cysteine molecules are adsorbed mainly as P(H) (gauche) conformer, in zwitterionic form with the COO- groups close to the surface. At more negative potentials, NH3+ groups deprotonate at the surface with simultaneous weakening of the interaction of the carboxylic groups with the surface. Spectroscopic evidence for at least partial protonation of the COO- groups at strongly acidic solutions was given by observing the C=O stretching band at frequency lowered by about 30 cm(-1) in comparison with that observed for crystalline cysteine hydrochloride. It points to the considerable enhancement of the strength of hydrogen bonds and may be ascribed to the formation of cyclic L-cysteine dimers at the electrode surface. In neutral and alkaline solutions, adsorbed L-cysteine molecules have deprotonated amino groups at wide potential range. Similar spectroelectrochemical experiments were performed for D-cysteine and for a racemic mixture of D,L-cysteine. As expected, results for D-cysteine were similar to those for L-cysteine. However, for racemic mixture at acidic pH, the spectral effects corresponding to potential-induced transition from adsorbed zwitterions to neutral molecule were considerably smaller. This effect was discussed in terms of stereoselective dimerization of cysteine molecule at the electrode surface.

pH and substrate effect on adsorption of peptides containing Z and E dehydrophenylalanine. surface-enhanced Raman spectroscopy studies on Ag nanocolloids and electrodes.

The silver substrates and pH dependent surface-enhanced Raman scattering (SERS) spectra of unsaturated derivatives of di- and tripeptides (dehydropeptides) are investigated. Experimental spectra were interpreted with the help of DFT calculations and normal-mode analysis. We choose as objects of our studies modified but natural peptides containing Z or E dehydrophenylalanine (((Z)/(E))ΔPhe) residue to study an effect of the type of the isomer on the interaction between the peptide and silver surfaces in the form of nanocolloidal particles and an electrochemically roughened electrode. We also observed that the SERS profile is sensitive to both the type of the studied SERS active substrate and pH, especially for the adsorption on the silver colloid. In general, all dehydropeptides interact with both SERS substrates upon deprotonation of the C-end of the molecule. The participation of the other fragments of the adsorbates such as the N-terminal amino group and the dehydroresidue is also manifested in the SERS spectra. Their orientation with respect to the silver surfaces is discussed in detail.

Comparative studies on IR, Raman, and surface enhanced Raman scattering spectroscopy of dipeptides containing ΔAla and ΔPhe.

Three dipeptides containing dehydroresidues (ΔAla, Δ((Z))Phe, and Δ((E))Phe) were examined by IR, Raman, and surface-enhanced Raman techniques for the first time. The effect of the size and isomer type of the ß-substituent in the dehydroresidue on the conformational structure of the peptide was evaluated by using the analysis of IR and Raman bands. Additionally, SERS spectroscopy provided insight into the adsorption mechanism of these species on the metal surface. SERS spectra were recorded at alkaline pH on the silver sol using visible light excitation. The dehydroresidues studied here strongly influenced the SERS profile of the peptides. The most pronounced SERS signal for all dipeptides was assigned to the symmetric stretching vibration of the carboxylate ions. This indicates that the dehydropeptides studied here primarily adsorb via the deprotonated carboxylic group. Additionally, the enhanced SERS bands in the range 1550-1650 cm(-1) show differences in contribution of the dehydroresidue to the adsorption mechanism of the studied peptides.

Structure and composition of binary monolayers self-assembled from sodium 2-mercaptoetanosulfonate and mercaptoundecanol mixed solutions on silver and gold supports.

Voltammetric reductive desorption, scanning tunneling microscopy (STM) and surface-enhanced Raman scattering (SERS) were used to determine the composition and structure of mixed, two-component monolayers of sodium 2-mercaptoetanosulfonate (MES) and mercaptoundecanol (MUL) on silver and gold supports. Monolayers were prepared by self assembling from ethanolic solutions of varying composition. Preferred adsorption of MUL was found in the electrochemical experiments on Au(111). The presence of two well-separated reductive desorption peaks in the voltammograms of the mixed monolayers on Au(111) indicated the existence of MES-rich and MUL-rich phases in a wide range of solution compositions. STM imaging confirmed formation of a few-nanometer-wide thiol domains for xMES greater than 0.5 in the solution used for SAMs preparation. On the contrary, SERS experiments pointed at dominant adsorption of MES on rough Ag and Au substrates. Nearly exclusive adsorption of the MES for xMES greater than 0.5 was observed on the rough Ag surface.

Properties of native and hydrophobic laccases immobilized in the liquid-crystalline cubic phase on electrodes.

Both native Trametes hirsuta laccase and the same laccase modified with palmytic chains to turn it more hydrophobic were prepared and studied with cyclic voltammetry and Raman spectroscopy. Native laccase immobilized in the monoolein cubic phase was characterized with resonance Raman spectroscopy, which demonstrated that the structure at the "blue" copper site of the protein remained intact. The diamond-type monoolein cubic phase prevents denaturation of enzymes on the electrode surface and provides contact of the enzyme with the electrode either directly or through the mediation by electroactive probes. Direct electron transfer for both laccases incorporated into a lyotropic liquid crystal was obtained under anaerobic conditions, whereas bioelectrocatalytic activity was shown only for the native enzyme. The differences in electrochemical behavior of native and hydrophobic laccase as well as possible mechanisms of direct and mediated electron transfers are discussed. The Michaelis constant for 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS2-), KMapp, and the maximal current, Imax, for the native enzyme immobilized onto the electrode were estimated to be 0.24 mM, and 5.3 microA, respectively. The maximal current density and the efficiency of the catalysis, Imax/KMapp, were found to be 73 microA cm-2 and 208.2 microA cm-2 mM-1, respectively, and indicated a high efficiency of oxygen electroreduction by the enzyme in the presence of ABTS2- in the cubic-phase environment. Rate constants were calculated to be 7.5x10(4) and 3.6x10(4) M-1 s-1 for native and hydrophobic laccase, respectively.