Nondestructive classification and identification of ballpoint pen inks by Raman spectroscopy for forensic document examinations

As a non-destructive analytical method, Raman spectroscopy often provides insufficient information to identify or differentiate the ink used for the preparation of a questioned document. In this study, blue and black ballpoint pen inks deposited on paper substrate were examined in situ by conventional Raman spectroscopy. Inks were successfully classified based on the total number of prominent bands in Raman spectra. It was found that more than 90% of the samples of the same type and color could be differentiated visually using only Raman spectra, i.e. 94 and 95% for blue and black inks, respectively. As a result of this study, a flow chart has been constructed for blue and black ballpoint pen inks allowing their systematic identification. Raman spectroscopy proved to be a fast and precise technique for forensic ink analysis.     

Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS

The authenticity of objects and artifacts is often the focus of forensic analytic chemistry. In document fraud cases, the most important objective is to determine the origin of a particular ink. Here, we introduce a new approach which utilizes the combination of two analytical methods, namely Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS). The methods provide complementary information on both molecular and elemental composition of samples. The potential of this hyphenation of spectroscopic methods is demonstrated for ten blue and black ink samples on white paper. LIBS and Raman spectra from different inks were fused into a single data matrix, and the number of different groups of inks was determined through multivariate analysis, i.e., principal component analysis, soft independent modelling of class analogy, partial least-squares discriminant analysis, and support vector machine. In all cases, the results obtained with the combined LIBS and Raman spectra were found to be superior to those obtained with the individual Raman or LIBS data sets.     

Applicability of surface-enhanced resonance Raman scattering for the direct discrimination of ballpoint pen inks

In situ surface-enhanced resonance Raman spectroscopy (SERRS) with excitation at 685 nm is suitable for the direct discrimination of blue and black ballpoint pen inks on paper. For black inks, shorter excitation wavelengths can also be used. For blue inks, SERRS at 514.5 and 457.9 nm does not provide adequate discriminative power. At these excitation wavelengths, the SERRS signals of the Methyl Violet derivatives present in inks easily dominate the overall spectrum because of resonance enhancement and preferential interaction with silver sol particles. At 685 nm, this problem is not encountered as the Methyl Violet derivatives do not show resonance enhancement, while other components may still exhibit resonance. Thirteen blue and thirteen black ink lines were examined. For the blue and black inks, on the basis of the 685 nm SERR spectra, eight and six groups of spectra, respectively, could be distinguished. This discrimination largely agrees with information from thin layer chromatography (TLC) experiments, although some differences in group compositions are found. The in situ SERR spectra show good repeatability with regard to the Raman frequencies, band shapes and relative intensities of the spectral bands. However, absolute intensities cannot be used for discrimination purposes.     

Nanoscale Chemical Imaging of Reversible Photoisomerization of an Azobenzene‐Thiol Self‐Assembled Monolayer by Tip‐Enhanced Raman Spectroscopy

An understanding of the photoisomerization mechanism of molecules bound to a metal surface at the molecular scale is required for designing photoswitches at surfaces. It has remained a challenge to correlate the surface structure and isomerization of photoswitches at ambient conditions. Herein, the photoisomerization of a self‐assembled monolayer of azobenzene‐thiol molecules on a Au surface was investigated using scanning tunneling microscopy and tip‐enhanced Raman spectroscopy. The unique signature of the cis isomer at 1525 cm^?1 observed in tip‐enhanced Raman spectra was clearly distinct from the trans isomer. Furthermore, tip‐enhanced Raman images of azobenzene thiols after ultraviolet and blue light irradiation are shown with nanoscale spatial resolution, demonstrating a reversible conformational change. Interestingly, the cis isomers of azobenzene‐thiol molecules were preferentially observed at Au grain edges, which is confirmed by density functional theory.     

Ion beam sputtering deposition of silver nanoparticles and TiOx/ZnO nanocomposites for use in surface enhanced vibrational spectroscopy (SERS and SEIRAS)

Hybrids consisting of silver nanoparticles (in varying fractions) and of TiOx/ZnO were prepared via top-down ion beam sputtering (IBS) deposition on silicon substrates. The deposited nanomaterials were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. It is shown that such composites represent a viable substrate for use in both surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRAS), as exemplarily shown for crystal violet as the model analyte. The C-H bending mode at about 1181 cm^?1 and the C-N vibration at 1361 cm^?1 observed in the SERS and SEIRAS spectra, respectively, have been used as analytical signal. The substrate consisting of TiOx NPs with 33% fraction of silver provides the strongest enhancement in SERS (up to 10,000-fold), while TiOx/AgNPs with thickness of 2 and 1 nm in ion beam sputtering, respectively, provides the best sensitivity in SEIRAS. The substrates also display photocatalytic activity as shown by the degradation of adsorbed crystal violet under ultraviolet irradiation.

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Graphical abstract Schematic of the preparation of hybrid substrates consisting of Ag and TiOx/ZnO nanoparticles via ion beam sputtering deposition. They were applied in both surface enhanced Raman and surface enhanced infrared absorption spectroscopies using crystal violet as model analyte, showing enhancements up to >10,000-fold in Raman.

     

脂质体@Ag/Au中空纳米壳层材料的制备及与H2O2的作用

以脂质体作为模板, 通过化学还原法制备了脂质体@Ag/Au 中空纳米材料, 并研究了其与过氧化氢(H₂O₂)的作用. 利用透射电子显微镜(TEM)、 紫外-可见(UV-Vis)吸收光谱和表面增强拉曼光谱(SERS)对纳米材料进行了表征. 在过氧化氢和纳米材料存在的情况下, 加入3,3 ',5,5'-四甲基联苯胺(TMB)作为显色液, 无色的TMB被氧化成蓝色的氧化TMB(oxTMB), 可以检测到反应后oxTMB在652 nm处的特征吸收强度及其拉曼光谱的变化. 实验结果表明, 当过氧化氢浓度在15100 μmol/L 范围内, oxTMB的特征吸收强度与其浓度呈线性关系, 可实现对过氧化氢的微量检测, 检测限为0.5 μmol/L.     

Surface enhanced raman and resonance raman spectroscopy in a non-aqueous electrochemical environment: Tris(2,2′-bipyridine)ruthenium(II) adsorbed on silver from acetonitrile

This letter reports the first observation of both surface enhanced Raman scattering (SERS) and surface enhanced resonance Raman scattering (SERRS) from the transition metal complex tris(2,2′-bipyridine)ruthenium (II), Ru(bpy)₃²⁺, adsorbed on a silver electrode from acetonitrile (ACN). The assignment of these spectra as valid examples of SERS and SERRS in a non-aqueous environment is based on the following criteria: (1) in situ demonstration of monolayer surface coverage of Ru(bpy)₃²⁺ using double potential step chronocoulometry (DPSCC); (2) the Raman signals are most intense after surface roughening by anodization; (3) the Raman spectra are potential dependent in the non-faradaic potential region; (4) the measured enhancement factors are greater ilian 10⁶; (5) the surface spectra are frequency shifted relative to their bulk counterpart; and (6) several other molecules also exhibit non-aqueous SERS and SERRS behavior. These results are highly significant in that generality of surface enhanced Raman spectroscopy has been extended into the rich domain of nonaqueous electrochemistry.     

Investigation of the metal adsorbate interface of the system silver coumarin and silver hydrocoumarin by means of surface enhanced Raman spectroscopy

Coumarin and hydrocoumarin adsorbed on silver island film were investigated by means of surface enhanced Raman spectroscopy. Polarized Raman spectra of the compounds were recorded and vibrational assignments for some characteristic Raman bands are given. The comparison of these spectra allows conclusions about the orientation of the molecule on the surface.     

Oxadiazole‐2‐thiol Adsorption on Gold Nanorods: A Joint Theoretical and Experimental Study by Using SERS,XPS, and DFT

The chemisorption of 1,3,4‐oxadiazole‐2‐thiol (ODT) on gold nanorods has been investigated by using surface‐enhanced Raman spectroscopy (SERS) and density functional theory (DFT). Although most of the SERS spectra have remarkable similarity to the normal Raman spectra of the pure analyte, the adsorption of ODT on a gold surface leads to a drastic change in its Raman spectrum and distinct vibrational features are obtained with gold nanorods and spherical nanoparticles. Simulated Raman spectra for hybrid systems that consist of an oxadiazole moiety coordinated to a Au₂₀ gold cluster provided valuable information about the coordination mode and enabled us to assign vibration modes.     

Design of a peptide linker group to increase the surface enhanced Raman spectroscopy signal intensity of a rhodamine-nanoparticle system

The surface enhanced resonance Raman spectra of three modified carboxy-x-rhodamine dyes were recorded using Au nanoparticles and an excitation laser operating at 670 nm. The dyes were modified with a linker group designed both to increase the surface enhanced Raman spectroscopy signal and to couple the dye to the Au nanoparticles surface. The maximum signal intensity was recorded for a Cys-Gly linker with Cys thiol group acting as the coupling point to the Au surface and Gly-NH₂ group used to attach the carboxy-x-rhodamine dye. This gave a signal intensity in the 1503 cm⁻¹ Raman peak that was more than 20 times greater than for the unmodified dye. The Au nanoparticles used had a diameter of 49.8 ± 1.2 nm and were synthesised by the citrate reduction method.     

The pH dependent Raman spectroscopic study of caffeine

First of all the surface enhanced Raman spectroscopy (SERS) and normal Raman spectra of caffeine aqueous solution were obtained at different pH values. In order to obtain the detailed vibrational assignments of the Raman spectroscopy, the geometry of caffeine molecule was optimized by density functional theory (DFT) calculation. By comparing the SERS of caffeine with its normal spectra at different pH values; it is concluded that pH value can dramatically affect the SERS of caffeine, but barely affect the normal Raman spectrum of caffeine aqueous solution. It can essentially affect the reorientation of caffeine molecule to the Ag colloid surface, but cannot impact the vibration of functional groups and chemical bonds in caffeine molecule.     

Phthalocyanine macrocycle as stabilizer for gold and silver nanoparticles

A one-step process was used for the preparation of gold and silver nanoparticles stabilized by an aminophthalocyanine macrocycle. The resultant nanoparticles were characterized by absorption spectra, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The nanoparticles were found to possess relatively narrow size distribution. The gold nanoparticles have an average diameter of ~2 nm, while silver particles have 4–5 nm. Preliminary studies on fluorescence and surface enhanced Raman spectroscopy were carried out using these nanoparticles. Fluorescence studies indicate that gold nanoparticles do not quench the fluorescence, while silver nanoparticles do. The stabilized nanoparticles showed enhancement of the Raman signals, thus revealing that they are good substrates for surface enhanced Raman scattering studies.     

Imaging Nanometre‐Sized Hot Spots on Smooth Au Films with High‐Resolution Tip‐Enhanced Luminescence and Raman Near‐Field Optical Microscopy

A novel near‐field optical microscope based on a parabolic mirror is used for recording high‐resolution tip‐enhanced photoluminescence (PL) and Raman images with unprecedented sensitivity and contrast. The measurements reveal small islands on the Au surface with dimensions of only a few nanometres with locally enhanced Au PL. These islands appear as nanometre‐sized hot spots in tip‐enhanced Raman microscopy when benzotriazole molecules adsorbed on the Au surface serve as local sensors for the optical field. The spectra show that localized plasmons are the cause of both the locally enhanced Au PL and enhanced Raman scattering. This finding suggests that the dispersive background in the surface‐enhanced Raman spectra can be explained simply by the enhanced Au PL in the gap. Furthermore, our results show that the surface flatness must be better than 1 nm, to provide an optically homogeneous substrate for near‐field enhanced PL and Raman spectroscopy.     

Concentration‐Dependent Hydrogen‐Bonding Effects on the Dimethyl Sulfoxide Vibrational Structure in the Presence of Water,Methanol, and Ethanol

The effects of hydrogen bonding between dimethyl sulfoxide (DMSO) and the co‐solvents water, methanol, and ethanol on the symmetric and antisymmetric CSC stretching vibrations of DMSO are investigated by means of Raman spectroscopy. The Raman spectra are recorded as a function of co‐solvent concentration and reflect changes in structure and polarizability as well as hydrogen‐bond donor and acceptor ability. In all cases studied a nonideal mixing behavior is observed. The spectra of the DMSO/water system show blue‐shifted CSC stretching modes. The antisymmetric frequencies are always further blue‐shifted than the symmetric stretching ones. The DMSO/methanol system also features blue‐shifted CSC stretching frequencies but at high mole fractions a pronounced red shifting is observed. In the binary DMSO/ethanol system, the co‐solvent also gives rise to blue shifts of the CSC stretching frequencies but restricted to mole fractions between x=0.38 and 0.45. The different magnitudes and occurrences of both blue‐ and red‐shifted spectral lines are comprehensively and critically discussed with respect to the existing literature concerning wavenumbers and Raman intensities in both absolute and normalized values. In particular, the normalized Raman intensities show a higher sensitivity for the nonideal mixing behavior because they are independent of the mole fraction.     

银/铂双金属中空纳米颗粒的制备及光学性质研究

通过以Ag纳米颗粒为模板的置换和沉积反应,制备了Ag/Pt双金属复合纳米颗粒.用透射电子显微镜(TEM)对颗粒的形貌、尺寸和结构进行了表征,发现复合颗粒具有中空结构.紫外可见吸收光谱(UV-Vis)研究表明,Ag/Pt双金属中空复合纳米颗粒具有单峰的表面等离子共振吸收特征,随着反应溶液中氯铂酸和硝酸银摩尔比的增加,吸收峰先红移后蓝移.表面增强拉曼光谱实验结果表明,Ag/Pt双金属复合纳米颗粒对吡啶分子具有较好的增强效果.     

Rapid monitoring of antibiotics using Raman and surface enhanced Raman spectroscopy

Comparatively few studies have explored the ability of Raman spectroscopy for the quantitative analysis of microbial secondary metabolites in fermentation broths. In this study we investigated the ability of Raman spectroscopy to differentiate between different penicillins and to quantify the level of penicillin in fermentation broths. However, the Raman signal is rather weak, therefore the Raman signal was enhanced using surface enhanced Raman spectroscopy (SERS) employing silver colloids. It was difficult by eye to differentiate between the five different penicillin molecules studied using Raman and SERS spectra, therefore the spectra were analysed by multivariate cluster analysis. Principal components analysis (PCA) clearly showed that SERS rather than the Raman spectra produced reproducible enough spectra to allow for the recovery of each of the different penicillins into their respective five groups. To highlight this further the first five principal components were used to construct a dendrogram using agglomerative clustering, and this again clearly showed that SERS can be used to identify which penicillin molecule was being analysed, despite their molecular similarities. With respect to the quantification of penicillin G it was shown that Raman spectroscopy could be used to quantify the amount of penicillin present in solution when relatively high levels of penicillin were analysed (>50 mM). By contrast, the SERS spectra showed reduced fluorescence, and improved signal to noise ratios from considerably lower concentrations of the antibiotic. This could prove to be advantageous in industry for monitoring low levels of penicillin in the early stages of antibiotic production. In addition, SERS may have advantages for quantifying low levels of high value, low yield, secondary metabolites in microbial processes.     

Fourier Transform Infrared Spectroscopy Applied to the Characterisation of 17th–20th Century Calcographic and Xylographic Inks

 The aim of this paper is to optimize the analytical method for the characterisation of 17th–20th century calcographic and xylographic inks. It is very important to use not only non destructive techniques, but also analytical instruments that require a minimal amount of material so as to analyze unique or valuable samples, and simultaneously to obtain as much information as possible about their composition. This study compares different ways of sample preparation for further analysis by Fourier transform infrared spectroscopy. It has been possible to analyze the most important absorptions that appear in the spectra and therefore to associate them with the compounds contained in inks. Some of the compounds identified are: hydroxyapatite (calcium phosphate), Prussian blue (ferric ferrocyanide), linseed oil, carbonates, nitrates and sulfates. Received January 10, 1999. Revision March 17, 1999.     

Microfluidic channel with embedded SERS 2D platform for the aptamer detection of ochratoxin A

A selective aptameric sequence is adsorbed on a two-dimensional nanostructured metallic platform optimized for surface-enhanced Raman spectroscopy (SERS) measurements. Using nanofabrication methods, a metallic nanostructure was prepared by electron-beam lithography onto a glass coverslip surface and embedded within a microfluidic channel made of polydimethylsiloxane, allowing one to monitor in situ SERS fingerprint spectra from the adsorbed molecules on the metallic nanostructures. The gold structure was designed so that its localized surface plasmon resonance matches the excitation wavelength used for the Raman measurement. This optofluidic device is then used to detect the presence of a toxin, namely ochratoxin-A (OTA), in a confined environment, using very small amounts of chemicals, and short data acquisition times, by taking advantage of the optical properties of a SERS platform to magnify the Raman signals of the aptameric monolayer system and avoiding chemical labeling of the aptamer or the OTA target.

NIR Raman spectra of whole human blood: effects of laser-induced and in vitro hemoglobin denaturation

Care must be exercised in the use of Raman spectroscopy for the identification of blood in forensic applications. The Raman spectra of dried whole human blood excited at 785 nm are shown to be exclusively due to oxyhemoglobin or related hemoglobin denaturation products. Raman spectra of whole blood are reported as a function of the incident 785-nm-laser power, and features attributable to heme aggregates are observed for fluences on the order of 10⁴ W/cm² and signal collection times of 20 s. In particular, the formation of this local-heating-induced heme aggregate product is indicated by a redshifting of several heme porphyrin ring vibrational bands, the appearance of a large broad band at 1,248 cm^-1, the disappearance of the Fe–O₂ stretching and bending bands, and the observation of a large overlapping fluorescence band. This denaturation product is also observed in the low-power-excitation Raman spectrum of older ambient-air-exposed bloodstains (2 weeks or more). The Raman spectrum of methemoglobin whole blood excited at 785 nm is reported, and increasing amounts of this natural denaturation product can also be identified in Raman spectra of dried whole blood particularly when the blood has been stored prior to drying. These results indicate that to use 785-nm-excitation Raman spectra as an identification method for forensic applications to maximum effect, incident laser powers need to be kept low to eliminate variable amounts of heme aggregate spectral components contributing to the signal and the natural aging process of hemoglobin denaturation needs to be accounted for. This also suggests that there is a potential opportunity for 785-nm-excitation Raman spectra to be a sensitive indicator of the age of dried bloodstains at crime scenes.