Surface-enhanced Raman spectroscopy studies of surfactant adsorption to a hydrophobic interface

Surface-enhanced Raman spectroscopy is used to investigate the kinetics of adsorption of the cationic surfactant cetylpyridinium chloride (CPC) to hydrophobic surfaces from water. A hydrophobic surface, with stable and reproducible SERS activity, is produced by binding gold colloids to an amine-terminated glass slide and then modifying this surface with octadecyltrimethoxysilane. In situ SERS-detected adsorption of CPC from aqueous solution is found to follow a Frumkin isotherm. Interactions between the charged head groups could be detected in frequency shifts in the symmetric ring breathing mode, consistent with an interfacial surfactant environment similar to a CPC micelle. Rates of surfactant adsorption were determined by time-resolved SERS measurements and were found to be much slower than the diffusion-controlled limit, indicating a significant kinetic barrier to adsorption. Desorption kinetics were heterogeneous, consistent with the spectroscopic results. Alkylsilane-modified gold colloids were shown to be useful substrates for investigating amphiphile adsorption from aqueous solutions to hydrophobic surfaces, where the adsorption kinetics could also be used to determine analyte concentrations in solution.     

表面增强拉曼光谱技术在纳米材料研究中的新进展

表面增强拉曼光谱是一种探测界面特性和分子之间相互作用、表征分子吸附行为和分子结构非常有效的现代测试分析工具.综述了表面增强拉曼的基本原理、发展历程以及在新型活性基底、碳纳米管、表面界面、单分子体系等研究领域中的应用,并展望了表面增强拉曼未来的发展方向.     

Raman spectroscopy-based creatinine measurement in urine samples from a multipatient population

Spectroscopic methods of urinalysis offer several advantages over chemical methods, including less sample contact and higher information content. In particular, urine creatinine has been the subject of several spectroscopic studies. We report the first use of Raman spectroscopy to measure creatinine concentrations in unaltered urine samples from a multipatient population. Using near-infrared excitation and a hybrid linear analysis calibration method, a root mean squared error of cross-validation (RMSECV) of 4.9 mg/dL was obtained. The error in the reference chemical method was 1.1 mg/dL. This result shows that the Raman spectroscopy can measure creatinine at clinical levels even in the presence of patient-to-patient variations. Because most assays in urine require creatinine concentration in order to correct for fluctuations in water content, measurement of creatinine is the first step towards more extensive Raman-based urinalysis.     

Surface-enhanced Raman scattering imaging of a single molecule on urchin-like silver nanowires

Urchin-like silver nanowires are prepared by reacting AgNO(3)(aq) with copper metal in the presence of cetyltrimethylammonium chloride and HNO(3)(aq) on a screen-printed carbon electrode at room temperature. The diameters of the nanowires are about 100 nm, and their lengths are up to 10 μm. Using Raman spectroscopy, the detection limit of Rhodamine 6G (R6G) on the urchin-like silver nanowire substrate can be as low as 10(-16) M, while the analytical enhancement factor is about 10(13). Raman mapping images confirm that a single R6G molecule on the substrate can be detected.     

Surface-enhanced Raman spectroscopy as a tool for probing specific biochemical components in bacteria

Treatment of bacteria with silver yields intense and highly specific surface-enhanced Raman spectroscopy (SERS) spectra from various cellular chemical components located in the vicinity of the silver colloids. In particular, we demonstrate an extreme sensitivity to flavin components associated with the cell envelope and to their state of oxidation. Different spectra, possibly associated with DNA, carboxylates, and perhaps phosphates, are obtained from the soluble interior fraction of the cell.     

Raman sensitivity enhancement for aqueous protein samples using a liquid-core optical-fiber cell

We have demonstrated a sensitivity enhancement factor of 500 in aqueous solutions using a liquid core optical fiber (LCOF) Raman cell made from Teflon-AF. We were able to collect a spectrum of 54 microM lysozyme with a signal-to-noise ratio of 31 in the LCOF Raman cell using 24 mW of laser power and 3 min of integration time. The lysozyme Raman intensity was only 1% of the background Raman intensity from water, but the water-subtracted lysozyme spectrum was still shot-noise-limited and essentially free of nonrandom noise. The lack of nonrandom noise indicates that it should be possible to collect good quality Raman spectra of proteins such as lysozyme at even lower concentrations. The 2.4-microL sample volume of the LCOF Raman cell is an added benefit when limited quantities of sample are available. This volume of a 54 microM lysozyme solution corresponds to only 13 nanomoles or 1.9 microg of lysozyme.     

Surface-enhanced Raman scattering substrate based on a self-assembled monolayer for use in gene diagnostics

The development of surface-enhanced Raman scattering (SERS)-active substrates for cancer gene detection is described. The detection method uses Raman active dye-labeled DNA gene probes, self-assembled monolayers, and nanostructured metallic substrates as SERS-active platforms. The mercaptohexane-labeled single-stranded DNA (SH-(CH(2))(6)-ssDNA)/6-mercapto-1-hexanol system formed on a silver surface is characterized by atomic force microscopy. The surface-enhanced Raman gene (SERGen) probes developed in this study can be used to detect DNA targets via hybridization to complementary DNA probes. The probes do not require the use of radioactive labels and have a great potential to provide both sensitivity and selectivity. The effectiveness of this approach and its application in cancer gene diagnostics (BRCA1 breast cancer gene) are investigated.     

Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS): a review of applications

Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) can provide positive identification of an analyte or an analyte mixture with high sensitivity and selectivity. Better understanding of the theory and advances in the understanding of the practice have led to the development of practical applications in which the unique advantages of SERS/SERRS have been used to provide effective solutions to difficult analytical problems. This review presents a basic theory and illustrates the way in which SERS/SERRS has been developed for practical use.     

Surface-enhanced Raman scattering based approach for quantitative determination of creatinine in human serum

A novel surface-enhanced Raman scattering (SERS) based approach for the quantitative determination of creatinine in human serum is described. Using isotopically labeled (2-13C, 2,3-15N2) creatinine as internal standard, SERS acquires the character of a ratio method that works similar to the well-established isotope dilution techniques. In conjunction with multivariate data analysis, the method was successfully applied for quantifying creatinine at clinically relevant levels and below. A partial least-squares regression model was generated from a set of 87 calibration spectra covering the full range of mole fractions of neat creatinine. The prediction performance of the model was thereafter validated with independent reference samples giving a standard deviation of less than 2%. Finally, a conditioning procedure to prepare real serum samples for SERS-based creatinine analysis was worked out and validated. Measured serum creatinine concentrations are within 3% of the values obtained from gas chromatography/isotope dilution mass spectrometry on the same serum starting material.     

One-step Synthesis of Shape-controllable Gold Nanoparticles and Their Application in Surface-enhanced Raman Scattering

Gold nanoparticles（NPs） of various shapes were synthesized by a one-step method at ambient temperature in the presence of NaCI.2-mercaptosuccinic acid（MSA） was used as both reducing agent and stabilizing agent.The shapes of gold NPs were controllable by simply tuning S/Au ratio（S is from MSA molecule,and S/Au ratio is controlled by tuning the volume of added MSA solution）,and triangle,polygonal and spherical nanoparticles were obtained.This result suggested a new way to consider the effects of MSA on the growth of nanoparticles,which showed that MSA is a strong capping agent and facilitates more uniform growth of nanoparticles in every dimension.And other important factors on nanoparticles growth including NaCI and temperature were discussed.Furthermore,a typical probe molecule,4-aminothiophenol（4-ATP） was used to test the surface-enhanced Raman scattering（SERS） activity of these gold NPs and the results indicated good Raman activity on these substrates.And the enhancement factor（EF） at 1078 cm^-1（a₁） was estimated to be as large as 6.3×10⁴ and 5.5×10⁴ for triangular plates and truncated particles,respectively.     

Tip-enhanced Raman scattering (TERS) from hemozoin crystals within a sectioned erythrocyte

Tip-enhanced Raman scattering (TERS) is a powerful technique to obtain molecular information on a nanometer scale, however, the technique has been limited to cell surfaces, viruses, and isolated molecules. Here we show that TERS can be used to probe hemozoin crystals at less than 20 nm spatial resolution in the digestive vacuole of a sectioned malaria parasite-infected cell. The TERS spectra clearly show characteristic bands of hemozoin that can be correlated to a precise position on the crystal by comparison with the corresponding atomic force microscopy (AFM) image. These are the first recorded AFM images of hemozoin crystals inside malaria-infected cells and clearly show the hemozoin crystals protruding from the embedding medium. TERS spectra recorded of these crystals show spectral features consistent with a five-coordinate high-spin ferric heme complex, which include the electron density marker band ν(4) at 1373 cm(-1) and other porphyrin skeletal and ring breathing modes at approximately 1636, 1557, 1412, 1314, 1123, and 1066 cm(-1). These results demonstrate the potential of the AFM/TERS technique to obtain nanoscale molecular information within a sectioned single cell. We foresee this approach paving the way to a new independent drug screening modality for detection of drugs binding to the hemozoin surface within the digestive vacuole of the malaria trophozoite.     

Infrared and Raman spectral signatures of aromatic nitration in thermoplastic urethanes

The spectral signatures of nitro attack of the aromatic portion of thermoplastic urethanes (TPU) were determined. Eight fragment molecules were synthesized that represent the nitrated and pristine methylenediphenyl section common to many TPUs. Infrared (IR) and Raman (785 nm illumination) spectra were collected and modeled using the B3LYP/6-31G(d)//B3LYP/6-31G(d) model chemistry. Normal mode animations were used to fully assign the vibrational spectra of each fragment. The vibrational assignment was used to develop a diagnostic method for aromatic nitro attack in thermoplastic urethanes. The symmetric NO(2) stretch coupled out of phase with the C-NO(2) stretch (1330 cm(-1)) was found to be free from spectral interferences. Spectral reference regions that enable correction for physical differences between samples were determined. The carbonyl stretch at 1700 cm(-1) was the best IR reference region, yielding a limit of quantitation (LOQ) of 0.66 +/- 0.02 g N/100 g Estane. Secondary IR reference regions were the N-H stretch at 3330 cm(-1) or the urethane nitrogen deformation at 1065 cm(-1). The reference region in the Raman was a ring stretching mode at 1590 cm(-1), giving an LOQ of 0.69 +/- 0.02 g N/100 g Estane. Raman spectroscopy displayed a larger calibration sensitivity (slope = 0.110 +/- 0.004) than IR spectroscopy (slope = 0.043 +/- 0.001) for nitration determination due to the large nitro Raman cross-section. The full spectral assignment of all eight molecules in the infrared and Raman is presented as supplemental material.     

Shift-excitation Raman difference spectroscopy-difference deconvolution method for the luminescence background rejection from Raman spectra of solid samples

The feasibility of the shift-excitation Raman difference spectroscopy-difference deconvolution (SERDS-DDM) method for fluorescence suppression from Raman spectra of solid samples is discussed. For SERDS measurements a tunable diode laser source with an emission band centered at 684 nm is coupled to a conventional micro-Raman apparatus and a monochromator device is used for checking the excitation frequency stability. The shifted Raman spectra are then mathematically treated and a deconvolution procedure is used to reconstruct the Raman spectrum devoid of fluorescence. Two different cases are presented. In the first one, fluorescence is intrinsic to the sample and the Raman spectrum of cinnabar pigment is finally reconstructed. In the second, the presence of an external luminescence background in the spectrum of a pure sulfur crystal is considered. The SERDS-DDM reconstructed spectra are compared with spectra obtained via multi-point baseline subtraction and a significant improvement in the detection of weak bands is demonstrated. Practical insights for the application of this method are presented as well.     

Surface-enhanced resonance Raman scattering of black inkjet dyes in solution and in situ printed onto paper

Understanding the changes that occur when dyes are absorbed onto paper is crucial for the design of new inkjet dyes. This problem is particularly difficult for black dyes that have complex chromophores, and as a result, spectroscopic information on electronic and structural changes can be of importance. Surface-enhanced resonance Raman scattering (SERRS) and electronic structure calculations were used to probe in situ changes in the chromophore in black di-azo dyes printed onto paper. The data indicate that the low-energy chromophore is due mainly to the hydrazone group and the high-energy chromophore to both the azo and hydrazone groups. A comparison of SERRS from the dyes adsorbed onto silver particles in suspension and from the dyes on paper demonstrated a broadening of the chromophore into the red for both dyes and evidence of a structural change in one dye.     

Quantitative analysis of Raman signal enhancement from aqueous samples in liquid core optical fibers

Raman scattering from aqueous liquids can be collected with high efficiency by enclosing the liquid within a suitable waveguide, as several groups have reported. Here, we present a quantitative model that predicts the relative strength of signals collected from (a) a tubular waveguide and (b) a flat-walled cuvette. Experimental measurements of Raman scattering from aqueous ethanol are made using two geometries, a Teflon-AF waveguide and a standard quartz cuvette. The model correctly predicts the enhancements in several ethanol Raman bands provided by the waveguide geometry. This model should be useful in aligning and characterizing liquid core waveguides, whose manufacture is still undergoing refinements. In particular, the model shows that absorption and scattering losses affect the enhancement factor in different ways.     

Analysis of tungsten in low grade ores and geological samples

The determination of tungsten in low grade ores and geological samples is one of the most difficult and challenging tasks. Many of the associated elements, especially molybdenum interfere. These have to be overcome by suitable methods of separation or suppression of the interfering elements. Since the concentrations are low, instrumental methods are preferred over the classical methods. Thus spectrophotometry, fluorometry, atomic absorption spectrometry, atomic emission spectrometry, X-ray fluorescence, neutron activation analysis, electro analytical methods and chromatography are preferred. These are discussed in this review.     

Surface-enhanced Raman scattering detection of DNA derived from the west nile virus genome using magnetic capture of Raman-active gold nanoparticles

A model paramagnetic nanoparticle (MNP) assay is demonstrated for surface-enhanced Raman scattering (SERS) detection of DNA oligonucleotides derived from the West Nile virus (WNV) genome. Detection is based on the capture of WNV target sequences by hybridization with complementary oligonucleotide probes covalently linked to fabricated MNPs and Raman reporter tag-conjugated gold nanoparticles (GNPs) and the subsequent removal of GNP-WNV target sequence-MNP hybridization complexes from solution by an externally applied magnetic source. Laser excitation of the pelleted material provided a signature SERS spectrum which is diagnostic for the reporter, 5,5'-dithiobis(succinimidy-2-nitrobenzoate) (DSNB), and restricted to hybridization reactions containing WNV target sequences. Hybridizations containing dilutions of the target oligonucleotide were characterized by a reduction in the intensification of the spectral peaks accorded to the SERS signaling of DSNB, and the limit of detection for target sequence in buffer was 10 pM. Due to the short hybridization times required to conduct the assay and ease with which reproducible Raman spectra can be acquired, the assay is amenable to adaptation within a portable, user-friendly Raman detection platform for nucleic acids.     

Multidimensional Raman spectroscopic signatures as a tool for forensic identification of body fluid traces: a review

The analysis of body fluid traces during forensic investigations is a critical step in determining the key details of a crime. Several confirmatory and presumptive biochemical tests are currently utilized. However, these tests are all destructive, and no single method can be used to analyze all body fluids. This review outlines recent progress in the development of a novel universal approach for the nondestructive, confirmatory identification of body fluid traces using Raman spectroscopy. The method is based on the use of multidimensional spectroscopic signatures of body fluids and accounts for the intrinsic heterogeneity of dry traces and donor variation. The results presented here demonstrate that Raman spectroscopy has potential for identifying traces of semen, blood, saliva, sweat, and vaginal fluid with high confidence.     

Surface-enhanced Raman scattering spectroscopy as a sensitive and selective technique for the detection of folic acid in water and human serum

Surface-enhanced Raman scattering (SERS) is shown to give linear and sensitive concentration-dependent detection of folic acid using silver nanoparticles created via ethylene-diaminetetraacetic acid (EDTA) reduction. Optical detection by SERS overcomes the primary limitation of photodissociation encountered during the application of other shorter wavelength ultraviolet (UV)/near-UV techniques such as fluorescence based microscopy. The SERS approach in water-based samples was demonstrated and optimized using several longer wavelengths of excitation (514.5, 632.8, and 785 nm). Excitation in the green (514.5 nm) was found to achieve the best balance between photodissociation and SERS efficiency. Linear concentration dependence was observed in the range of 0.018 to 1 microM. The importance of folic acid in a clinical setting and the potential applications of this technique in a biological environment are highlighted. We demonstrate the potential to transfer this technique to real biological samples by the detection of folic acid in human serum samples by SERS.     

Ultrasensitive PCR and real-time detection from human genomic samples using a bidirectional flow microreactor

In this paper we present a reliable bidirectional flow DNA amplification microreactor for processing real-world genomic samples. This system shares the low-power thermal responsiveness of a continuous flow reactor with the low surface area to volume ratio character of stationary reactors for reducing surface inhibitory effects. Silanization with dimethyldichlorosilane in combination with dynamic surface passivation was used to enhance PCR compatibility and enable efficient amplification. For real-time fragment amplification monitoring we have implemented an epimodal fluorescent detection capability. The passivated bidirectional flow system was ultrasensitive, achieving an RNase P gene detection limit of 24 human genome copies with a reaction efficiency of 77%. This starts to rival the performance of a conventional real-time PCR instrument with a reaction efficiency of 93% and revitalizes flow-through PCR as a viable component of lab on a chip DNA analysis formats.