Surface-enhanced Raman scattering (SERS) revealing chemical variation during biofilm formation: from initial attachment to mature biofilm

Surface-enhanced Raman scattering (SERS) has recently been proved to be a promising technique for characterizing the chemical composition of the biofilm matrix. In the present study, to fully understand the chemical variations during biofilm formation, SERS based on silver colloidal nanoparticles was applied to evaluate the chemical components in the matrix of biofilm at different growth phases, including initial attached bacteria, colonies, and mature biofilm. Meanwhile, atomic force microscopy was also applied to study the changes of biofilm morphology. Three model bacteria, including Escherichia coli, Pseudomonas putida, and Bacillus subtilis, were used to cultivate biofilms. The results showed that the content of carbohydrates, proteins, and nucleic acids in the biofilm matrix increased significantly along with the biofilm growth of the three bacteria judging from the intensities and appearance probabilities of related marker peaks in the SERS spectra. The content of lipids, however, only increased in the Gram-negative biofilms (E. coli and P. putida) rather than the Gram-positive biofilm (B. subtilis). Our findings strongly suggest the SERS has significant potential for studying chemical variations during biofilm formation.     

Preparation of Fe3O4@Ag SERS substrate and its application in environmental Cr(VI) analysis

A novel sensitive and recyclable SERS substrate which can actively concentrate chromate (Cr(VI)) in water and substantially enhance Raman signal was synthesized as uniform Fe(3)O(4)@Ag nanoparticles. The surface morphology, structure, and magnetic properties were characterized using transmission electron microscopy, atomic force microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and vibrating sample magnetometry analysis. The closely spaced Fe(3)O(4)@Ag substrate with a core-shell structure exhibited a 25 nm surface roughness. The high saturation magnetization at 48.35 emu g(-1) enabled the complete and rapid separation of the substrate from the solution. The sensitivity and reproducibility of the substrate were confirmed using a common SERS probe molecule, rhodamine 6G. SERS spectra of Cr(VI) in simulated and real contaminated water showed that the symmetric stretching vibrations of Cr-O occurred at 796 cm(-1). This SERS peak area exhibited a linear dependence (R(2)=0.9992) on the Cr(VI) concentration between 5 and 100 μg L(-1). Coexisting anions such as sulfate, nitrate, chloride, carbonate, and humic acid could decrease the sensitivity of the SERS analysis. However, the adverse effect of the competing ions may be eliminated by proper dilution of the raw sample. This study provides a reliable method for qualitative and quantitative analysis of Cr(VI).     

Surface enhanced Raman spectroscopy (SERS) for the discrimination of Arthrobacter strains based on variations in cell surface composition

Surface enhanced Raman spectroscopy (SERS) is a rapid and highly sensitive spectroscopic technique that has the potential to measure chemical changes in bacterial cell surface in response to environmental changes. The objective of this study was to determine whether SERS had sufficient resolution to differentiate closely related bacteria within a genus grown on solid and liquid medium, and a single Arthrobacter strain grown in multiple chromate concentrations. Fourteen closely related Arthrobacter strains, based on their 16S rRNA gene sequences, were used in this study. After performing principal component analysis in conjunction with Linear Discriminant Analysis, we used a novel, adapted cross-validation method, which more faithfully models the classification of spectra. All fourteen strains could be classified with up to 97% accuracy. The hierarchical trees comparing SERS spectra from the liquid and solid media datasets were different. Additionally, hierarchical trees created from the Raman data were different from those obtained using 16S rRNA gene sequences (a phylogenetic measure). A single bacterial strain grown on solid media culture with three different chromate levels also showed significant spectral distinction at discrete points identified by the new Elastic Net regularized regression method demonstrating the ability of SERS to detect environmentally induced changes in cell surface composition. This study demonstrates that SERS is effective in distinguishing between a large number of very closely related Arthrobacter strains and could be a valuable tool for rapid monitoring and characterization of phenotypic variations in a single population in response to environmental conditions.     

The role of hydrogen bonding in the selectivity of L-cysteine methyl ester (CYSM) and L-cysteine ethyl ester (CYSE) for chloride ion

The interaction of cysteamine (CY), L-cysteine methyl ester (CYSM), and L-cysteine ethyl ester (CYSE) with nitrate, sulfate, perchlorate, dihydrogen phosphate, and chloride ions was investigated using surface enhanced Raman spectroscopy (SERS). CYSM and CYSE are chemical derivatives of CY. These thiols have a quaternary ammonium group to attract the anions to the SERS surface. Dihydrogen phosphate did not interact with these cationic thiols. The CY interaction with perchlorate, nitrate, and sulfate is stronger than the interaction with chloride. However, replacing a hydrogen on the carbon adjacent to the quaternary ammonium group with either a methyl or ethyl ester group results in stronger complexation with chloride ion than with either sulfate or nitrate ion. In the case of CYSM, the chloride interaction is five times stronger than the interaction with perchlorate. Molecular modeling indicates that the high selectivity of CYSM/CYSE for chloride is due to hydrogen bonding between the chloride ion and the hydrogen of the CH3 moeities of adjacent ester groups.     

表面增强拉曼光谱在细菌耐药性评价方面的研究进展

细菌耐药性问题引发全球关注。表面增强拉曼光谱技术(SERS)凭借灵敏度高、检测速度快等优势在评价细菌耐药性应用方面备受关注。本文首先总结了与细菌耐药性评价相关的SERS基底及检测方法,然后对SERS光谱技术在耐药菌和敏感菌鉴定、细菌生物膜成膜性分析与评价及抗菌药物敏感性筛查方面的应用进行总结,最后对SERS技术在细菌检测方面的一些瓶颈问题展开了讨论。希望本文能为SERS技术在细菌耐药性评价方面的应用提供方法指导和思路借鉴。     

Bimetallic gold–silver nanoplate array as a highly active SERS substrate for detection of streptavidin/biotin assemblies

The silver-modified gold nanoplate arrays as bimetallic surface-enhanced Raman scattering (SERS) substrates were optimized for the surface-enhanced Raman detection of streptavidin/biotin monolayer assemblies. The bimetallic gold–silver nanoplate arrays were fabricated by coating silver nanoparticles uniformly on the gold nanoplate arrays. Depending on silver nanoparticle coating, the localized surface plasmon resonance (LSPR) peak of the bimetallic gold–silver nanoplate arrays blue-shifted and broadened significantly. The common probe molecule, Niel Blue A sulfate (NBA) was used for testing the SERS activity of the bimetallic gold–silver nanoplate arrays. The SERS intensity increased with the silver nanoparticle coating, due to a large number of hot spots and nanoparticle interfaces. The platforms were tested against a monolayer of streptavidin functionalized over the bimetallic gold–silver nanoplate arrays showing that good quality spectra could be acquired with a short acquisition time. The supramolecular interaction between streptavidin (strep) and biotin showed subsequent modification of Raman spectra that implied a change of the secondary structure of the host biomolecule. And the detection concentration for biotin by this method was as low as 1.0 nM. The enhanced SERS performance of such bimetallic gold–silver nanoplate arrays could spur further interest in the integration of highly sensitive biosensors for rapid, nondestructive, and quantitative bioanalysis, particularly in microfluidics.     

Comparative study of various methods of the heterogeneous reduction of nitrate ions

Working pH ranges of heterogeneous reductors of nitrate ions based on metallic cadmium, zinc, and brass as well as on cadmium-copper, cadmium-mercury, zinc-copper, and zinc-mercury galvanic couples were examined. Metallic cadmium and the cadmium-copper couple were selected as reductants; freshly prepared, these reductants provided the 100% reduction of nitrate ions to nitrite ions in the pH range from 3.4 to 9.5. A method was developed for prolonging the reducing properties of reductants based on metallic cadmium by permanently copper-plating these reductants. The relationship between the degree of reduction of nitrate ions in a cadmium reductor and the concentration of Cd²⁺ ions in the solution at the outlet of the reductor was determined.     

Monitoring metal ion binding in single-layer Pseudomonas aeruginosa biofilms using ATR-IR spectroscopy

The binding of metal ions to Pseudomonas aeruginosa PAO1 cells attached to a ZnSe surface has been observed in this research through cation exchange experiments using ATR-IR spectroscopy. A biofilm consisting of a single layer of Pseudomonas aeruginosa PAO1 cells was formed on a ZnSe prism by flowing a bacterial suspension in a 0.03 mol L(-)(1) NaNO(3) solution at pH 5.0 across its surface. Exposure of the biofilm to chromium(III) nitrate solution resulted in increases in all band absorbances. This absorbance increase has been attributed to the binding of chromium(III) to the bacterial exopolymers associated with the prism surface. The chromium(III) binding causes the exopolymers to contract and move the bacterial cell closer to the ZnSe surface. Further study of chromium(III) ion exchange using a mutant P. aeruginosa with a truncated lipopolysaccharide (LPS) chain resulted in much smaller absorbance changes. This observation supports the view that the extension of bacterial exopolymers and hence the distance of the bacterial cell from the surface is strongly influenced by environmental factors such as the presence of metal cations. Following chromium(III) cation exchange, the bacterial band absorbances remained constant even when the bacteria were washed with a 0.03 mol L(-)(1) NaNO(3) solution, indicating that the chromium(III) was irreversibly bound. Ion exchange with nickel(II) and cobalt(II) nitrate solutions within identical biofilms showed that these cations caused relatively small increases in absorbances that were reversible, indicating that nickel(II) and cobalt(II) are less strongly bound than chromium(III) within P. aeruginosa biofilms. The absence of discernible IR spectral changes with metal binding appears to indicate a predominantly electrostatic mechanism for binding of Cr(III), Ni(II), and Co(II) ions by bacteria in the early stages of biofilm formation.     

Stability of silver colloids as substrate for surface enhanced Raman spectroscopy detection of dipicolinic acid

Silver colloids have been commonly used as substrates for surface enhanced Raman spectroscopy (SERS). It has been shown that SERS requires partial aggregation of the silver colloids. This study evaluates factors affecting the aggregative state of the silver colloids such as the age of the silver colloids and the aggregation as a result of addition of the analyte. The silver colloids are obtained from the chemical reduction of silver nitrate by sodium borohydride. Further oxidation of the sodium borohydride solution at room temperature results in concentration changes of the resulting silver colloids. Methods of controlling the sodium borohydride depletion are presented in this paper. The analyte used is dipicolinic acid, a molecular signature of Bacillus spores.     

Comparative study of the morphology, aggregation, adherence to glass, and surface-enhanced Raman scattering activity of silver nanoparticles prepared by chemical reduction of Ag+ using citrate and hydroxylamine

Two different silver colloids were prepared by chemical reduction of silver nitrate with trisodium citrate and hydroxylamine hydrochloride to compare their characteristics in relation to their possible use in surface-enhanced Raman scattering (SERS) spectroscopy. The morphology and plasmon resonance of the single nanoparticles and aggregates integrating these colloids were characterized by means of UV-vis absortion spectroscopy and scanning electron microscopy, revealing important differences between each type of nanoparticle as concerns their physical properties. These metallic systems also manifested differences in the aggregation and the adherence to glass surfaces, revealing significant differences in the chemical surface properties of these nanoparticles. SERS and surface-enhanced IR also indicated the presence of interference bands which can overlap the spectra of the analyte, mainly in the case of the citrate colloid. All these differences have an important influence on the applicability of these nanostructured systems in SERS. In fact, the enhancement factor and spectral pattern of the SERS obtained by using alizarin as a molecule probe are different.     

Electroreduction of chromic acid during the chromium deposition: An RRDE study

The rate of the trivalent chromium ion formation during the chromic acid reduction in the presence of sulfate ions is studied in a wide potential range by the RRDE method. At potentials of incomplete reduction of chromate ions, the reaction is attendant by a side process (probably, the hydrogen ion reduction). With a phase film covering the disk cathode, the ring current does not correspond to the number of Cr(III) ions formed at the disk, rather it characterizes the oxidation of products of a phase film comprising oxide-hydroxide compounds of Cr(III). The hypothesis about the formation of a phase cathodic film, presumably preceding the metallic chromium deposition, is confirmed experimentally. The oxide-hydroxide species of Cr(III), which are main film-forming substances, undergo phase transitions in a zone that depends on the sulfate ion concentration. The phase transitions whose products may reach the ring electrode occur in the near-electrode layer. The lower the sulfate ion concentration, the larger the extension of the layer.     

Recent Advances in Metal Organic Frameworks Based Surface Enhanced Raman Scattering Substrates: Synthesis and Applications

Metal-organic frameworks (MOFs) are supramolecular nanomaterials, in which metal ions or clusters are connected by organic ligands to form crystalline lattices with highly ordered periodic porous network structure. MOFs have been widely applied in various fields, such as catalyst, sample preparation, and sensing. In recent years, MOFs based surface enhanced Raman scattering (SERS) substrates have attracted much attention since MOFs can largely improve the performance of metallic SERS substrates toward target enrichment and signal enhancement. MOFs have been exploited in SERS analysis to tackle some challenges that bare metal substrates cannot achieve. Combination of MOFs and SERS improved the sensitivity of traditional SERS analysis and extended the application scope of SERS. With the increasing exploration of MOFs based SERS substrates, there is a great demand to review the advances in these researches. Herein, this review concentrated on summarizing the preparation and applications of MOFs based SERS substrates. Representative researches were discussed to better understand the property of MOFs based SERS substrates. The advantages of MOFs based SERS substrates were highlighted, as well as their limitations. In addition, the challenges, opportunities, and future trends in MOFs based SERS analysis were tentatively discussed.     

Silver–platinum core–shell nanoparticles for surface-enhanced Raman spectroscopy

Core–shell Ag@Pt nanoparticles have been synthesised by the means of seed-growth reaction including reduction of PtCl₄²⁻ with silver and replacing Ag atoms with Pt. Surface-enhanced Raman scattering (SERS) spectra of pyridine (which gives slightly different spectra when interacting with various metals) adsorbed on synthesised Ag@Pt clusters were measured. SERS measurements have revealed that deposition of the platinum layer causes near elimination of the spectral interferences from pyridine directly interacting with the silver core. The average SERS enhancement factor for pyridine adsorbed on the Ag@Pt clusters was estimated as equal to about 10³–10⁴, significantly higher than the SERS enhancement factor achievable on the pure platinum nanostructures. Using the silver core (instead of the previously used gold cores) allows for measurement of strong SERS spectra on the Pt covered nanostructures for the wider range of the excitation radiation. This procedure of platinum deposition was tested with various silver nanoparticles – produced with borohydride, citrate and citrate/borohydride methods – which substantially differ in size distribution. The application of formed Ag@Pt structures for obtaining intense Raman spectra for molecules adsorbed on only slightly modified platinum surfaces is discussed.     

Reproducible discrimination between gram-positive and gram-negative bacteria using surface enhanced Raman spectroscopy with infrared excitation

The on time diagnostics of bacterial diseases is one of the essential steps in the foregoing treatment of such pathogens. Here we sought to present an easy to use and robust method for the discrimination between Gram-positive (Enterococcus faecalis and Streptococcus pyogenes) and Gram-negative (Acinetobacter baumannii and Klebsiella pneumoniae) bacterial genera based on surface enhanced Raman scattering (SERS) spectroscopy. The robustness of our approach lies in the novel method for the production of the SER substrate based on silver nanoparticles and their subsequent re-crystallization in solutions containing high concentrations of chloride ions. The method presented here could be an interesting alternative both to commonly used histochemical approaches and commercial SERS substrates.     

Synthesis of positively charged silver nanoparticles via photoreduction of AgNO3 in branched polyethyleneimine/HEPES solutions

Branched polyethyleneimine (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were used collaboratively to reduce silver nitrate under UV irradiation for the synthesis of positively charged silver nanoparticles. The effects of molar ratio of the ingredients and the molecular weight of BPEI on the particle size and distribution were investigated. The mechanism for the reduction of Ag+ ions in the BPEI/HEPES mixtures entails oxidative cleavage of BPEI chains that results in the formation of positively charged BPEI fragments enriched with amide groups as well as in the production of formaldehyde, which serves as a reducing agent for Ag+ ions. The resultant silver nanoparticles are positively charged due to protonation of surface amino groups. Importantly, these positively charged Ag nanoparticles demonstrate superior SERS activity over negatively charged citrate reduced Ag nanoparticles for the detection of thiocyanate and perchlorate ions; therefore, they are promising candidates for sensing and detection of a variety of negatively charged analytes in aqueous solutions using surface-enhanced Raman spectroscopy (SERS).     

Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications

This tutorial review discusses a new class of colloidal metal nanoparticles that is able to enhance the efficiencies of surface-enhanced Raman scattering (SERS) by as much as 10(14)-10(15) fold. This enormous enhancement allows spectroscopic detection and identification of single molecules located on the nanoparticle surface or at the junction of two particles under ambient conditions. Considerable progress has been made in understanding the enhancement mechanisms, including definitive evidence for the single-molecule origin of fluctuating SERS signals. For applications, SERS nanoparticle tags have been developed based on the use of embedded reporter molecules and a silica or polymer encapsulation layer. The SERS nanoparticle tags are capable of providing detailed spectroscopic information and are much brighter than semiconductor quantum dots in the near-infrared spectral window. These properties have raised new opportunities for multiplexed molecular diagnosis and in vivo Raman spectroscopy and imaging.     

等离激元激励表面增强拉曼光谱检测技术与仪器

本文总结了近年来基于传播型表面等离激元（Propagafingsurfaceplasmons,PSPs）参与的表面增强拉曼（Surface—enhancedRamanscattering,SERS）技术和仪器方面的研究进展．内容主要包括3部分：（1）基于PSPs激励拉曼散射的装置和技术,包括在消逝场下激发PSPs共振增强拉曼的原理与装置、与表面等离子体共振（Surfaceplasmonresonance,SPR）传感技术的联用及新型结构的长程等离激元激励拉曼技术的研究进展;（2）通过引入局域型表面等离激元（Localizedsurfaceplasmons,LSPs）进一步增强SERS,进而实现PSPs-LSPs共同增强拉曼的超灵敏检测技术,包括在消逝场激发的PSPs基础上,增加纳米粒子实现的PSPs与LSPs共同增强拉曼的原理、装置,以及用该方法进行生物体系的免疫识别检测,此外,还在微纳周期结构上实现了PSPs与LSPs共同激励拉曼;（3）基于PSPs耦合的定向SERS技术,包括在消逝场结构和周期结构上实现SERS定向耦合发射以达到高激发和高收集效率的新技术．     

Polymeric Nanoparticles Active against Dual-Species Bacterial Biofilms

Biofilm infections are a global public health threat, necessitating new treatment strategies. Biofilm formation also contributes to the development and spread of multidrug-resistant (MDR) bacterial strains. Biofilm-associated chronic infections typically involve colonization by more than one bacterial species. The co-existence of multiple species of bacteria in biofilms exacerbates therapeutic challenges and can render traditional antibiotics ineffective. Polymeric nanoparticles offer alternative antimicrobial approaches to antibiotics, owing to their tunable physico-chemical properties. Here, we report the efficacy of poly(oxanorborneneimide) (PONI)-based antimicrobial polymeric nanoparticles (PNPs) against multi-species bacterial biofilms. PNPs showed good dual-species biofilm penetration profiles as confirmed by confocal laser scanning microscopy. Broad-spectrum antimicrobial activity was observed, with reduction in both bacterial viability and overall biofilm mass. Further, PNPs displayed minimal fibroblast toxicity and high antimicrobial activity in an in vitro co-culture model comprising fibroblast cells and dual-species biofilms of Escherichia coli and Pseudomonas aeruginosa. This study highlights a potential clinical application of the presented polymeric platform.     

Surface-enhanced Raman scattering-active silver nanostructures with two domains

Generally, a controllable and reproduced surface roughness for surface-enhanced Raman scattering (SERS) studies can be generated through control of the electrochemical oxidation–reduction cycles (ORC) procedure. In this work, we propose a new sonoelectrochemical approach to prepare SERS-active substrates with two domain-Ag nanostructures. The method is based on a strategy of deposition–dissolution cycles (DDCs) by using a cathodic overpotential and an anodic overpotential from open circuit potential (OCP) in turn under sonication. The prepared SERS-active substrate demonstrates large Raman scattering enhancement for adsorbed Rhodamine 6G (R6G) with an enhancement factor of 2.3 × 10⁸ and a limit of detection of 2 × 10⁻¹³ M. The improved SERS performances can be successfully explained from the viewpoints of electromagnetic (EM) and chemical (CHEM) enhancements.     

Surface-enhanced Raman spectrometry of amiloride on colloidal silver

Surface-enhanced Raman spectrometry (SERS) of the diuretic drug amiloride is discussed. The SERS-active substrate used is colloidal silver, which is prepared at room temperature by simple tetrahydroborate reduction of an aqueous solution of silver nitrate. SERS detection of amiloride in human urine is illustrated. The reproducibility of quantitative SERS data is supported by the simultaneous measurement of the scattering signal and transmitted light. Relative standard deviations at the 5% level are compatible with many practical analytical situations.