SERS study on adenine using a Ag/poly(t-butylacrylate) nanocomposite

Surface-Enhanced Raman Scattering (SERS) Substrates Based on Ag-Nanoparticles and Ag-Nanoparticles/Poly (methyl methacrylate) Composites

SERS substrates formed by spherical silver nanoparticles (Ag-NPs) with a 15 nm average diameter adsorbed on Si substrate at three different concentrations and Ag/PMMA composites formed by an opal of PMMA microspheres of 298 nm average diameter were synthesized. The Ag-NPs were varied at three different concentrations. We have observed from SEM micrographs, in the Ag/PMMA composites, the periodicity of the PMMA opals is slightly altered as the Ag-NP concentration is increased; as a consequence of this effect, the PBGs maxima shift toward longer wavelengths, decrease in intensity, and broaden as the Ag-NP concentration is increased in the composites. The performance of single Ag-NP and Ag/PMMA composites as SERS substrates was determined using methylene blue (MB) as a probe molecule with concentrations in the range of 0.5 µM to 2.5 µM. We found that in both single Ag-NP and Ag/PMMA composites as SERS substrates, the enhancement factor (EF) increases as the Ag-NP concentration is increased. We highlight that the SERS substrate with the highest concentration of Ag-NPs has the highest EF due to the formation of metallic clusters on the surface, which generates more "hot spots". The comparison of the EFs of the single Ag-NP with those of Ag/PMMA composite SERS substrates shows that the EFs of the former are nearly 10-fold higher than those of Ag/PMMA composites. This result is obtained probably due to the porosity of the PMMA microspheres that decreases the local electric field strength. Furthermore, PMMA exerts a shielding effect that affects the optical efficiency of Ag-NPs. Moreover, the metal-dielectric surface interaction contributes to the decrease in the EF. Other aspect to consider in our results is in relation to the difference in the EF of the Ag/PMMA composite and Ag-NP SERS substrates and is due to the existing mismatch between the frequency range of the PMMA opal stop band and the LSPR frequency range of the Ag metal nanoparticles adsorbed on the PMMA opal host matrix.

Surface-enhanced Raman scattering of DNA bases using frozen silver nanoparticle dispersion as a platform

Raman spectroscopy is a powerful method to characterize molecules in various media. Although surface-enhanced Raman scattering (SERS) is often employed to compensate for the intrinsically poor sensitivity of Raman spectroscopy, there remain serious tasks, such as simple preparations of SERS substrates, sensitivity control, and reproducible measurements. Here, we propose freezing as an efficient way to overcome these problems in SERS measurements using DNA bases as model targets. Solutes are expelled from ice crystals and concentrated in the liquid phase upon freezing. Silver nanoparticles (AgNPs) are also concentrated in the liquid phase to aggregate with Raman target analytes. The SERS signal intensity is maximized when the AgNP concentration exceeds the critical aggregation value. Freezing allows up to 5000 times enhancements of the SERS signal. Thus, an efficient SERS platform is prepared by simple freezing. The simultaneous detection of four DNA bases effectively eliminates variations of signal intensities and allows the reliable determination of concentration ratios.

Silver-Based SERS Pico-Molar Adenine Sensor

Adenine is an important molecule for biomedical and agricultural research and applications. The detection of low concentration adenine molecules is thus desirable. Surface-enhanced Raman scattering (SERS) is a promising label-free detection and fingerprinting technique for molecules of significance. A novel SERS sensor made of clusters of silver nanostructures deposited on copper bumps in valleys of an etched silicon substrate was previously reported to exhibit a low and reproducible detection limit for a 10⁻¹¹ M neutral adenine aqueous solution. Reflection of laser illumination from the silicon surface surrounding a valley provides additional directions of laser excitation to adenine molecules adsorbing on a silver surface for the generation of enhanced SERS signal strength leading to a low detection limit. This paper further reports a concentration dependent shift of the ring-breathing mode SERS adenine peak towards 760 cm⁻¹ with decreasing concentration and its pH-dependent SERS signal strength. For applications, where the pH value can vary, reproducible detection of 10⁻¹² M adenine in a pH 9 aqueous solution is feasible, making the novel SERS structure a desirable pico-molar adenine sensor.

Novel SERS-based process analysis for label-free segmented flow screenings

In microfluidic segmented flow processes label-free analytical techniques like surface enhanced Raman spectroscopy (SERS) can reveal the chemical composition of the individual droplet contents. The SERS system developed in this work enables a simple connection to micro segmented flow processes through miniaturization. The concept is based on the parallelization of silver/polyacrylamide composite SERS spots on a carrier plate on which the segments are deposited. The transfer of the segments allows an easy connection to existing flow processes and provides optimal conditions for Raman measurements using miniaturized spectrometers. The preparation of the SERS polymer composite was optimized in terms of the silver content in the polymer matrix to obtain a high SERS signal. The performance and long-term stability of the polymer have been successfully demonstrated. The deamination of adenine with sodium nitrite to hypoxanthine was chosen as a case study to demonstrate the capability of the novel SERS-based process analysis. A sequence of approximately one hundred segments in combination with a gradient of the nitrite concentration (0 to 0.4 mol L-1) was generated at two temperatures. The concentration of adenine and hypoxanthine were determined by using a multivariate calibration model, since the Raman spectra of both substances are overlapping. It was shown that the conversion of adenine is increased with higher nitrite concentration and temperature. A conversion of 35% was obtained at 50 °C and a conversion of 60% at 80 °C, respectively.

Multi-hydrogen bond assisted SERS detection of adenine based on multifunctional graphene oxide/poly (diallyldimethyl ammonium chloride)/Ag nanocomposites

Nanocomposites of graphene oxide/poly (diallyldimethyl ammonium chloride)/Ag nanoparticles (GO/PDDA/Ag NPs) were constructed via a self-assembly process as a surface-enhanced Raman scattering (SERS) substrate, in which functional macromolecules PDDA were utilized to load GO and support Ag NPs. Fundamental SERS performance of this SERS substrate was evaluated using rhodamine 6G (R6G), which displayed excellent enhancement effect, transferable nature and high stability of the synthesized GO/PDDA/Ag NPs substrate. Furthermore, the synthesized SERS substrate was employed in the sensitive detection of adenine with a linear range of 0.05-1000 μM and low detection limit of 1 nM. Other than the large surface area of GO, multiple-hydrogen bond interactions between adenine and the modified PDDA were another important factor in capturing adenine molecules and enhancing SERS signal. The hydrogen bond interaction was calculated using quantum mechanical calculations. Moreover, determination of adenine in aqueous solutions was achieved with good anti-interference ability against other nucleic bases with similar structures, such as guanine, cytosine and thymine. Therefore, GO/PDDA/Ag can be anticipated to be a potential substrate for label-free, fast and sensitive SERS detection of adenine in the field of bioanalysis.

Energetics and Vibrational Signatures of Nucleobase Argyrophilic Interactions

This study investigates the interactions of both purine (adenine and guanine) and pyrimidine (cytosine, thymine, and uracil) nucleobases with a pair of silver atoms (Ag2). Full geometry optimizations were performed on several structures of each nucleobase/Ag2 complex and the corresponding isolated monomers using the M06-2X density functional with a correlation consistent triple-ζ basis set augmented with diffuse functions on all atoms and a relativistic pseudopotential on Ag (aug-cc-pVTZ for H, C, N, and O and aug-cc-pVTZ-PP for Ag; denoted aVTZ). Harmonic vibrational frequency computations indicate that each optimized structure corresponds to a minimum on the M06-2X/aVTZ potential energy surface. Relative electronic energies for interactions between Ag2 and each nucleobase were compared to elucidate energetic differences between isomers. Further analysis of the changes in vibrational frequencies, infrared intensities, and Raman scattering activities reveals how different Ag2 binding sites might be differentiated spectroscopically. These results provide molecular-level insight into the interactions between nucleobases and silver, which may lead to better understanding and interpretation of surface-enhanced Raman spectroscopy experiments on nucleobases and related systems.

Applications of silver nanoparticles stabilized and/or immobilized by polymer matrixes

Nanomaterials frequently possess unique and noticeably changed physical, chemical and biological properties compared to their macro scaled corresponding item. Utilization of nanoparticles habitually requires the construction of integrated chemical systems. Most popular of these are polymer-supported nanoparticles. In this review, we provide the reader with the last developments and breakthrough technologies concerning silver nanoparticles (AgNPs), one of the most comprehensively studied nanomaterials, considering the polymer types and processes used for the nanocomposite membranes preparation.

Hybrid nanostructures for SERS: materials development and chemical detection

This review focuses on recent developments in hybrid and nanostructured substrates for SERS (surface-enhanced Raman scattering) studies. Thus substrates composed of at least two distinct types of materials, in which one is a SERS active metal, are considered here aiming at their use as platforms for chemical detection in a variety of contexts. Fundamental aspects related to the SERS effect and plasmonic behaviour of nanometals are briefly introduced. The materials described include polymer nanocomposites containing metal nanoparticles and coupled inorganic nanophases. Chemical approaches to tailor the morphological features of these substrates in order to get high SERS activity are reviewed. Finally, some perspectives for practical applications in the context of chemical detection of analytes using such hybrid platforms are presented.

Analysis of polycyclic aromatic hydrocarbons in water with gold nanoparticles decorated hydrophobic porous polymer as surface-enhanced Raman spectroscopy substrate

A method for surface-enhanced Raman spectroscopy (SERS) sensing of polycyclic aromatic hydrocarbons (PAHs) is reported. Gold nanoparticles (AuNPs) decorated hydrophobic porous glycidyl methacrylate-ethylene dimethacrylate (GMA-EDMA) polymer is developed as the SERS substrate. GMA-EDMA material with porosity and permeability shows rapid and efficient adsorption of PAHs through presumed hydrophobic interaction, which brings the analytes close to the substrate. Meanwhile, the three dimensional porous morphology might benefit AuNPs distribution for high SERS enhancement. Studies on the effects of AuNPs surface coverage on the substrate and time of PAHs-substrate interaction are presented. The qualitative analysis and quantitative tendency of this method for PAHs detection are investigated with anthracene, phenanthrene and pyrene as probe molecules, showing that the characteristic fingerprint vibrational peaks of each PAH can be readily identified, and the limit of detections are 0.93×10(-7), 4.5×10(-7) and 1.1×10(-7) M respectively. Moreover, the substrate exhibits high reproducibility with the relative standard deviation about 16% in spot and spot SERS intensity. Using this method for rapid screening of PAHs mixture in some water samples are performed well, which might be useful for environmental pollutions monitoring.

Composite blends of gold nanorods and poly(t-butylacrylate) beads as new substrates for SERS

Polymer based composites containing metal nanoparticles are shown to provide new substrates for SERS detection and simultaneously enable the development of new tools for molecular sensing. A very important aspect on the use of hybrid materials relates to the observation of synergistic effects that result from the use of distinct components such as a polymer and metal nanoparticles. We report here the preparation of new SERS substrates made from blends of colloidal Au (nanospheres and nanorods) and PtBA (poly(t-butylacrylate)). The observed SERS enhancement depends on the characteristics of the obtained hybrid material. When compared to the starting Au colloids, the Au/PtBA nanocomposites led to SERS spectra with more intense bands, using 2,2'-dithiodipyridine as molecular probe. The origin of the stronger Raman signal in this case, is possibly due to a combination of events related to the nanocomposite characteristics, including the formation of Au assemblies at the polymer surface due to particle clustering, and the absorption increase in a spectral region closer to the laser excitation wavelength. The strategy described here is a low-cost process with potential for the up-scale fabrication of SERS substrates, namely by using other types of nanocomposites.