Silver overlayer-modified surface-enhanced Raman scattering-active gold substrates for potential applications in trace detection of biochemical species

Because Ag and Au nanoparticles (NPs) possess well-defined localized surface plasmon resonance (LSPR) they are popularly employed in the studies of surface-enhanced Raman scattering (SERS). As shown in the literature and in our previous studies, the advantage of SERS-active Ag NPs is their higher SERS enhancement over Au NPs. On the other hand, the disadvantage of SERS-active Ag NPs compared to Au NPs is their serious decay of SERS enhancement in ambient laboratory air. In this work, we develop a new strategy for preparing highly SERS-active Ag NPs deposited on a roughened Au substrate. This strategy is derived from the modification of electrochemical underpotential deposition (UPD) of metals. The coverage of Ag NPs on the roughened Au substrate can be as high as 0.95. Experimental results indicate that the SERS of Rhodamine 6G (R6G) observed on this developed substrate exhibits a higher intensity by ca. 50-fold of magnitude, as compared with that of R6G observed on the substrate without the deposition of Ag NPs. The limit of detection (LOD) for R6G measured on this substrate is markedly reduced to 2 × 10⁻¹⁵ M. Moreover, aging of SERS effect observed on this developed substrate is significantly depressed, as compared with that observed on a generally prepared SERS-active Ag substrate. These aging tests were performed in an atmosphere of 50% relative humidity (RH) and 20% (v/v) O₂ at 30 °C for 60 day. Also, the developed SERS-active substrate enables it practically applicable in the trace detection of monosodium urate (MSU)-containing solution in gouty arthritis without a further purification process.     

Using a photochemical method and chitosan to prepare surface-enhanced Raman scattering–active silver nanoparticles

In this paper, we report a new strategy for the preparation of surface-enhanced Raman scattering (SERS)-active silver nanoparticles (Ag NPs), using a photochemical method and the presence of chitosan (Ch). First, Ag substrates were subjected to electrochemical oxidation/reduction cycles (ORCs) in deoxygenated aqueous solutions containing 0.1 M HNO₃ and 1 g L⁻¹ Ch (pH 6.9, adjusted by adding 1 M NaOH), resulting in Ag⁺–Ch complexes. These substrates were then irradiated with UV light at various wavelengths to yield the SERS-active Ag NPs. A stronger SERS effect was observed on the SERS-active Ag NPs prepared by using UV irradiation at 310 nm. The pH of the solution and the presence of Ch during the preparation process both affected the resulting SERS activities.     

Fabrication of bimetallic microfluidic surface-enhanced Raman scattering sensors on paper by screen printing

Au–Ag bimetallic microfluidic, dumbbell-shaped, surface enhanced Raman scattering (SERS) sensors were fabricated on cellulose paper by screen printing. These printed sensors rely on a sample droplet injection zone, and a SERS detection zone at either end of the dumbbell motif, fabricated by printing silver nanoparticles (Ag NPs) and gold nanoparticles (Au NPs) successively with microscale precision. The microfluidic channel was patterned using an insulating ink to connect these two zones and form a hydrophobic circuit. Owing to capillary action of paper in the millimeter-sized channels, the sensor could enable self-filtering of fluids to remove suspended particles within wastewater without pumping. This sensor also allows sensitive SERS detection, due to advantageous combination of the strong surface enhancement of Ag NPs and excellent chemical stability of Au NPs. The SERS performance of the sensors was investigated by employing the probe rhodamine 6G, a limit of detection (LOD) of 1.1 × 10⁻¹³ M and an enhancement factor of 8.6 × 10⁶ could be achieved. Moreover, the dumbbell-shaped bimetallic sensors exhibited good stability with SERS performance being maintained over 14 weeks in air, and high reproducibility with less than 15% variation in spot-to-spot SERS intensity. Using these dumbbell-shaped bimetallic sensors, substituted aromatic pollutants in wastewater samples could be quantitatively analyzed, which demonstrated their excellent capability for rapid trace pollutant detection in wastewater samples in the field without pre-separation.     

Highly reproducible surface-enhanced Raman scattering-active Au nanostructures prepared by simple electrodeposition: Origin of surface-enhanced Raman scattering activity and applications as electrochemical substrates

The fabrication of effective surface-enhanced Raman scattering (SERS) substrates has been the subject of intensive research because of their useful applications. In this paper, dendritic gold (Au) rod (DAR) structures prepared by simple one-step electrodeposition in a short time were examined as an effective SERS-active substrate. The SERS activity of the DAR surfaces was compared to that of other nanostructured Au surfaces with different morphologies, and its dependence on the structural variation of DAR structures was examined. These comparisonal investigations revealed that highly faceted sharp edge sites present on the DAR surfaces play a critical role in inducing a high SERS activity. The SERS enhancement factor was estimated to be greater than 10⁵, and the detection limit of rhodamine 6G at DAR surfaces was 10⁻⁸ M. The DAR surfaces exhibit excellent spot-to-spot and substrate-to-substrate SERS enhancement reproducibility, and their long-term stability is very good. It was also demonstrated that the DAR surfaces can be effectively utilized in electrochemical SERS systems, wherein a reversible SERS behavior was obtained during the cycling to cathodic potential regions. Considering the straightforward preparation of DAR substrates and the clean nature of SERS-active Au surfaces prepared in the absence of additives, we expect that DAR surfaces can be used as cost-effective SERS substrates in analytical and electrochemical applications.     

Optofluidic microsystem with quasi-3 dimensional gold plasmonic nanostructure arrays for online sensitive and reproducible SERS detection

Practical applications of chemical and biological detections through surface-enhanced Raman scattering (SERS) require high reproducibility, sensitivity, and efficiency, along with low-cost, straightforward fabrication. In this work, we integrated a poly-(dimethylsiloxane) (PDMS) chip with quasi-3D gold plasmonic nanostructure arrays (Q3D-PNAs), which serve as SERS-active substrates, into an optofluidic microsystem for online sensitive and reproducible SERS detections. The Q3D-PNA PDMS chip was fabricated through soft lithography to ensure both precision and low-cost fabrication. The optimal dimension of the Q3D-PNA in PDMS was designed using finite-difference time-domain (FDTD) electromagnetic simulations with a simulated enhancement factor (EF) of 1.6 × 10⁶. The real-time monitoring capability of the SERS-based optofluidic microsystem was investigated by kinetic on/off experiments through alternatively flowing Rhodamine 6G (R6G) and ethanol in the microfluidic channel. A switch-off time of ∼2 min at a flow rate of 0.3 mL min⁻¹ was demonstrated. When applied to the detection of low concentration malathion, the SERS-based optofluidic microsystem with Q3D-PNAs showed high reproducibility, significantly improved efficiency and higher detection sensitivity via increasing the flow rate. The optofluidic microsystem presented in this paper offers a simple and low-cost approach for online, label-free chemical and biological analysis and sensing with high sensitivity, reproducibility, efficiency, and molecular specificity.     

Ag Nanoparticles Decorated CuO@RF Core-Shell Nanowires for High-Performance Surface-Enhanced Raman Spectroscopy Application

Vertical-aligned CuO nanowires have been directly fabricated on Cu foil through a facile thermal oxidation process by a hotplate at 550 °C for 6 h under ambient conditions. The intermediate layer of resorcinol–formaldehyde (RF) and silver (Ag) nanoparticles can be sequentially deposited on Cu nanowires to form CuO@RF@Ag core-shell nanowires by a two-step wet chemical approach. The appropriate resorcinol weight and silver nitrate concentration can be favorable to grow the CuO@RF@Ag nanowires with higher surface-enhanced Raman scattering (SERS) enhancement for detecting rhodamine 6G (R6G) molecules. Compared with CuO@Ag nanowires grown by ion sputtering, CuO@RF@Ag nanowires exhibited a higher SERS enhancement factor of 5.33 × 10⁸ and a lower detection limit (10⁻¹² M) for detecting R6G molecules. This result is ascribed to the CuO@RF@Ag nanowires with higher-density hot spots and surface-active sites for enhanced high SERS enhancement, good reproducibility, and uniformity. Furthermore, the CuO@RF@Ag nanowires can also reveal a high-sensitivity SERS-active substrate for detecting amoxicillin (10⁻¹⁰ M) and 5-fluorouracil (10⁻⁷ M). CuO@RF@Ag nanowires exhibit a simple fabrication process, high SERS sensitivity, high reproducibility, high uniformity, and low detection limit, which are helpful for the practical application of SERS in different fields.     

Improved surface-enhanced Raman scattering on optimum electrochemically roughened silver substrates

In this work, the effects of preparation conditions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) were first investigated. The optimum roughening conditions for obtaining strongest SERS of Rhodamine 6G (R6G) are as follows. Ag electrodes were cycled in deoxygenated aqueous solutions containing 0.1 M NaCl from −0.3 to +0.2 V versus Ag/AgCl at 25 mV s⁻¹ for five scans. The SERS of R6G adsorbed on this optimum procedure-prepared roughened Ag substrate exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a normally roughened Ag substrate.     

Bare gold nanoparticles mediated surface-enhanced Raman spectroscopic determination and quantification of carboxylated single-walled carbon nanotubes

The paper proposes a simple and portable approach for the surface enhanced Raman scattering (SERS) spectroscopy in situ determination of carboxylated single walled carbon nanotubes (SWNTs) in river water samples. The method is based on the subsequent microfiltration of a bare gold nanoparticles solution and the water sample containing soluble carbon nanotubes by using a home-made filtration device with a small filtration diameter. An acetate cellulose membrane with a pore size of 0.2 μm first traps gold nanoparticles to form the SERS-active substrate and then concentrates the carbon nanotubes. The measured SERS intensity data were closely fit with a Langmuir isotherm. A portable Raman spectrometer was employed to measure SERS spectra, which enables in situ determination of SWNTs in river waters. The limit of detection was 10 μg L⁻¹. The precision, for a 10 mg L⁻¹ concentration of carbon nanotubes, is 1.19% intra-membrane and 10.5% inter-membrane.     

Determination of 6-mercaptopurine based on the fluorescence enhancement of Au nanoparticles

A novel method for the determination of 6-mercaptopurine (6MP) has been developed based on fluorescence enhancement of Au nanoparticles (AuNPs). The fluorescent AuNPs with mean diameter of ∼15 nm were synthesized in aqueous solution, exhibiting the stable maximum emission at 367 nm, under the excitation at wavelength of 264 nm. The AuNPs self-assembly with 6MP were characterized with transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption, fluorescence and surface-enhanced Raman scattering (SERS) spectroscopy. The results revealed that the surface attachment through versatile binding sites of S10, N3, N9 and N7 atoms in 6MP produced the interparticle coupling and formed aggregates of AuNPs. As a result, the fluorescence emission enhancement was significantly observed upon AuNPs self-assembly with 6MP. The fluorimetric determination under optimal conditions indicated that 6MP could be quantified in good linearity range of 6.35 × 10⁻⁸ to 3.05 × 10⁻⁷ M, with a low detection limit of 4.82 × 10⁻¹⁰ M. The relative standard deviation (n = 11) was 1.8% at 2.54 × 10⁻⁸ M 6MP concentration level. The proposed method was successfully applied for the determination of 6MP in spiked human urine. The probable fluorescence enhancement mechanism was also discussed there.     

Improved surface-enhanced Raman scattering on electrochemically roughened silver substrates prepared in bielectrolyte solutions

In this work, the effect of supplemental LiClO₄ electrolytes in KCl solutions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) was first investigated. To prepare SERS-active substrates by ORC procedures, electrolytes of KCl were generally employed. In contrast, LiClO₄ ones were unsuitable for producing SERS-active substrates. Encouragingly, SERS of Rhodamine 6G (R6G) adsorbed on the roughened Ag substrate prepared in an aqueous solution containing KCl and LiClO₄ electrolytes exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a roughened Ag substrate prepared in a solution only containing KCl. Further investigations indicate that the oxidation state of Cl on the roughened Ag substrate demonstrates decided effects on this improved SERS.     

Electromagnetic and chemical interaction between Ag nanoparticles and adsorbed rhodamine molecules in surface-enhanced Raman scattering

The critical importance of the junction between touching or closely adjacent Ag nanoparticles associated with single-molecule sensitivity (SMS) in surface-enhanced Raman scattering (SERS) was confirmed via the following observations: (1) an additional peak is observed in elastic scattering only for the SERS-active state, which originated from absorption of adsorbates, (2) local- and far-field evaluation using a finite difference time domain method could reproduce this extra peak and anticipate the significantly enhanced field even inside the adsorbates sitting at the junction through an increased coupling of the localized surface plasmons, and (3) in addition to enhanced fluorescence of adsorbed dye, an inelastic scattering peak was observed and attributed to the metal surface electron. Concerning the chemical enhancement in SERS, Cl⁻ anions activate the Ag-Cl-R6G (rhodamine) samples by inducing intrinsic electronic interaction between Ag and R6G molecules. This electronic interaction is irreversibly quenched by the addition of thiosulfate anions which dissolve Ag⁺ cations while the electromagnetic (EM) effect remains intact.     

A new strategy to prepare surface-enhanced Raman scattering-active substrates by electrochemical pulse deposition of gold nanoparticles

In this communication, we develop a simple pathway to prepare SERS-active substrates with Au nanoparticles (NPs) by an electrochemical strategy of deposition-dissolution cycles (DDCs). The prepared SERS-active substrates demonstrate large Raman scattering enhancement for Rhodamine 6G (R6G) with a detection limit of 2 × 10(-12) M and an enhancement factor of 5.8 × 10(7).     

Coating of silver nanoparticles (AgNPs) on glass fibers by a chemical method as plasmonic surface-enhanced Raman spectroscopy (SERS) sensors to detect molecular vibrations of Doxorubicin (DOX) drug in blood plasma

In the present study, Doxorubicin (DOX) drug in healthy blood plasma was the focus of the investigation by surface-enhanced Raman scattering (SERS). In recent years, chemotherapy has been the most popular treatment for various types of cancer; however, its adverse side effects on the patient's health have made a negative aspect regarding the use of this technique. DOX is the most common chemotherapy drug and is used for the treatment of an extensive range of human malignancies. The surface-enhanced Raman scattering (SERS) is a precise technique for the detection of chemicals and biomaterials with significantly low concentrations. The glass fiber substrates coated with silver nanoparticles (AgNPs) have been used to detect DOX. First, the Tollens' method was applied to prepare the AgNPs, and the characteristics of fabricated AgNPs were evaluated using ultraviolet–visible spectroscopy (UV–Vis) and X-ray diffraction (XRD). Then, AgNPs were coated on the glass fiber substrate by a chemical method. Finally, the enhancement of the Raman signal resulted from the molecular vibrations of DOX was evaluated using these SERS-active substrates as plasmonic and Raman spectroscopy sensors. Afterward, for making the sensors practical, the DOX in blood plasma were deposited on the fabricated sensors, and the Raman vibrations were evaluated. The SERS-active substrates, AgNPs deposited on glass fiber substrates, were fabricated for the detection of DOX in and out of the blood plasma; the limit of detection (LOD) for both was 10^?10 M, and the mean relative standard deviation at concentrations of 10^?10 M of DOX out of blood plasma, and 10^?10 M of DOX in blood plasma were obtained to be 3.76% and 3.61%, respectively for ten repeated measurements in which the AgNPs were SERS-active substrates of the biosensors for detecting the DOX. In addition, the enhancement factor was calculated both experimentally and via finite-difference time-domain (FDTD) simulation, which was 29.76 × 10³ and 24.95 × 10³, respectively. Therefore, these SERS-active substrates can be used to develop microsensors and show positive results for SERS-based investigations.     

Synthesis of Fe3O4\SiO2\Ag nanoparticles and its application in surface-enhanced Raman scattering

To obtain a recyclable surface-enhanced Raman scattering (SERS) material, we developed a composite of Fe₃O₄\SiO₂\Ag with core\shell\particles structure. The designed particles were synthesized via an ultrasonic route. The Raman scattering signal of Fe₃O₄ could be shielded by increasing the thickness of the SiO₂ layer to 60 nm. Dye rhodamine B (RB) was chosen as probe molecule to test the SERS effect of the synthesized Fe₃O₄\SiO₂\Ag particles. On the synthesized Fe₃O₄\SiO₂\Ag particles, the characteristic Raman bands of RB could be observed when the RB solution was diluted to 5 ppm (1×10⁻⁵ M). Furthermore, the synthesized particles could keep their efficiency till four cycles.     

Surface-imprinted core–shell Au nanoparticles for selective detection of bisphenol A based on surface-enhanced Raman scattering

Surface-imprinted core–shell Au nanoparticles (AuNPs) were explored for the highly selective detection of bisphenol A (BPA) by surface-enhanced Raman scattering (SERS). A triethoxysilane-template complex (BPA-Si) was synthesized and then utilized to fabricate a molecularly imprinted polymer (MIP) layer on the AuNPs via a sol–gel process. The imprinted BPA molecules were removed by a simple thermal treatment to generated the imprint-removed material, MIP-ir-AuNPs, with the desired recognition sites that could selectively rebind the BPA molecules. The morphological and polymeric characteristics of MIP-ir-AuNPs were investigated by transmission electron microscopy and Fourier-transform infrared spectroscopy. The results demonstrated that the MIP-ir-AuNPs were fabricated with a 2 nm MIP shell layer within which abundant amine groups were generated. The rebinding kinetics study showed that the MIP-ir-AuNPs could reach the equilibrium adsorption for BPA within 10 min owning to the advantage of ultrathin core–shell nanostructure. Moreover, a linear relationship between SERS intensity and the concentration of BPA on the MIP-ir-AuNPs was observed in the range of 0.5–22.8 mg L⁻¹, with a detection limit of 0.12 mg L⁻¹ (blank ± 3 × s.d.). When applied to SERS detection, the developed surface-imprinted core–shell MIP-ir-AuNPs could recognize BPA and prevent interference from the structural analogues such as hexafluorobisphenol A (BPAF) and diethylstilbestrol (DES). These results revealed that the proposed method displayed significant potential utility in rapid and selective detection of BPA in real samples.     

A surface-enhanced Raman scattering method for detection of trace glutathione on the basis of immobilized silver nanoparticles and crystal violet probe

Unsatisfactory sensitivity and stability for molecules with low polarizability is still a problem limiting the practical applications of surface-enhanced Raman scattering (SERS) technique. By preparing immobilized silver nanoparticles (Fe₃O₄/Ag) through depositing silver on the surface of magnetite particles, a highly sensitive and selective SERS method for the detection of trace glutathione (GSH) was proposed on the basis of a system of Fe₃O₄/Ag nanoparticles and crystal violet (CV), in which the target GSH competed with the CV probe for the adsorption on the Fe₃O₄/Ag nanoparticles. Raman insensitive GSH replaced the highly Raman sensitive CV adsorbed on the surface of Fe₃O₄/Ag particles. This replacement led to a strong decrease of the CV SERS signal, which was used to determine the concentration of GSH. Under optimal conditions, a linear response was established between the intensity decrease of the CV SERS signal and the GSH concentration in the range of 50–700 nmol L⁻¹ with a detection limit of 40 nmol L⁻¹. The use of a Fe₃O₄/Ag substrate provided not only a great SERS enhancement but also a good stability, which guarantees the reproducibility of the proposed method. Its use for the determination of GSH in practical blood samples and cell extract yielded satisfactory results.     

二步电沉积法制备Au/氧化石墨烯复合薄膜作为SERS基底

采用二步电沉积方法在Ti片表面制备了Au-氧化石墨烯（Au-GO）复合薄膜,通过XRD、SEM、XPS等对薄膜的组成、结构和形貌进行了表征,并以罗丹明6G（R6G）为探针分子,对Au-GO/Ti基底的SERS活性进行了表征。结果显示,Au纳米颗粒尺寸约为60 nm,均匀、致密分布于GO表面,该基底显示出较高的SERS活性,对R6G分子的检测极限可达~10^-10 mol·L^-1,增强因子高达约10⁶,且基底显示出良好的稳定性,在冰箱中存放90 d后,SERS活性仅降低30%左右。     

二步电沉积法制备Au/氧化石墨烯复合薄膜作为SERS基底

采用二步电沉积方法在Ti片表面制备了Au-氧化石墨烯（Au-GO）复合薄膜,通过XRD、SEM、XPS等对薄膜的组成、结构和形貌进行了表征,并以罗丹明6G（R6G）为探针分子,对Au-GO/Ti基底的SERS活性进行了表征。结果显示,Au纳米颗粒尺寸约为60 nm,均匀、致密分布于GO表面,该基底显示出较高的SERS活性,对R6G分子的检测极限可达~10^-10 mol·L^-1,增强因子高达约10⁶,且基底显示出良好的稳定性,在冰箱中存放90 d后,SERS活性仅降低30%左右。     

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.     

Novel fabrication of silver-coated glass capillaries for ready SERS-based detection of dissolved chemical species

A novel method to deposit a highly surface-enhanced Raman scattering (SERS) active silver film onto the inside surface of a glass capillary is developed. Firstly, Ag sol was synthesized by the reaction of AgNO₃ with poly-(ethylenimine) (PEI), and then toluene and benzenethiol (BT) were added into the sol. The mixture was flowed through the glass capillary to obtain the SERS-active Ag film-coated glass capillary. The SERS activity of the Ag-coated capillary was dependent on the amount of PEI and BT used. In addition, BT could be easily desorbed from the Ag surface by treating it with a borohydride solution, maintaining the initial SERS activity. The SERS enhancement factor at 632.8-nm excitation was estimated to be on the order of 10⁶. The detection limits of adenine and dipicolinic acid were then as low as 1.0 × 10⁻⁸ and 1.0 × 10⁻⁷ M, respectively, based on an S/N ratio of 3. This clearly suggests that the Ag-coated capillary is an invaluable device for the analysis of effluent chemicals by SERS.