We detected concentration-dependent surface-enhanced Raman scattering (SERS) spectra of several label-free proteins (lysozyme, ribonuclease B, avidin, catalase, and hemoglobin) for the first time in aqueous solutions. Acidified sulfate was used as an aggregation agent to induce high electromagnetic enhancement in SERS. Strong SERS spectra of simple and conjugated protein samples could easily be accessed after the pretreatment with the aggregation agent. The detection limits of the proposed method for lysozyme and catalase were as low as 5 microg/mL and 50 ng/mL, respectively. This detection protocol for label-free proteins has combined simplicity, sensitivity, and reproducibility and allows routine qualitative and relatively quantitative detections. Thus, it has great potential in practical high-throughput protein detections.

In this paper, a direct and simple detection method based on surface-enhanced Raman scattering (SERS) named "heat-induced SERS sensing method" is proposed for rapid determination of glutathione in aqueous solutions. It was found that highly enhanced SERS spectra of glutathione can be obtained if the silver colloids adsorbed with the analyte were heated up before the SERS measurement. Besides, it was revealed that silver particles with a size of approximately 60 nm are suitable for this study and that the SERS intensity is also influenced by the dropped sample volume, drying temperature, buffer concentration, and pH of the solution. It is noted that the thiol group of glutathione has a particularly strong interaction with a silver surface compared with other small biological molecules without a thiol group, validating this method to detect glutathione selectively. Under the optimal conditions, the detection of glutathione can be finished within 5 min, and the detection limit of ca. 50 nM can be reached, which is much better than the reported detection limit of glutathione (approximately 1 microM) by SERS. The enhancement factor of the proposed heat-induced SERS sensing method for the detection of glutathione is about 7.5 x 10(6). The proposed method holds a specific selectivity toward glutathione, facilitating its rapid detection in practical applications.

Surface-enhanced Raman scattering (SERS) enhancement and the reproducibility of the SERS signal strongly reflect the quality and nature of the SERS substrates because of diverse localized surface plasmon resonance (LSPR) excitations excited at interstitials or sharp edges. LSPR excitations are the most important ingredients for achieving huge enhancements in the SERS process. In this report, we introduce several gold and silver nanoparticle-based SERS-active substrates developed solely by us and use these substrates to investigate the influence of LSPR excitations on SERS. SERS-active gold substrates were fabricated by immobilizing colloidal gold nanoparticles on glass slides without using any surfactants or electrolytes, whereas most of the SERS-active substrates that use colloidal gold/silver nanoparticles are not free of surfactant. Isolated aggregates, chain-like elongated aggregates and two-dimensional (2D) nanostructures were found to consist mostly of monolayers rather than agglomerations. With reference to correlated LSPR and SERS, combined experiments were carried out on a single platform at the same spatial position. The isolated aggregates mostly show a broadened and shifted SPR peak, whereas a weak blue-shifted peak is observed near 430 nm in addition to broadened peaks centered at 635 and 720 nm in the red spectral region in the chain-like elongated aggregates. In the case of 2D nanostructures, several SPR peaks are observed in diverse frequency regions. The characteristics of LSPR and SERS for the same gold nanoaggregates lead to a good correlation between SPR and SERS images. The elongated gold nanostructures show a higher enhancement of the Raman signal than the the isolated and 2D samples. In the case of SERS-active silver substrates for protein detection, a new approach has been adopted, in contrast to the conventional fabrication method. Colloidal silver nanoparticles are immobilized on the protein functionalized glass slides, and further SERS measurements are carried out based on LSPR excitations. A new strategy for the detection of biomolecules, particularly glutathione, under aqueous conditions is proposed. Finally, supramolecular J-aggregates of ionic dyes incorporated with silver colloidal aggregates are characterized by SERS measurements and correlated to finite-difference time-domain analysis with reference to LSPR excitations.

The heterogeneous oxidation reaction of single aqueous ascorbic acid (AH2) aerosol particles with gas-phase ozone was investigated in this study utilizing aerosol optical tweezers with Raman spectroscopy. The measured liquid-phase...

Recently, synthesis, characterization, and application of carbon dots have received much attention. Natural products are the effectual carbon precursors to synthesize carbon dots with fascinating chemical and physical properties. In this study, the fluorescent sensor of carbon dots derived from cranberry beans without any functionalization and modification was developed. The carbon dots were prepared with a cheap, facile, and green carbon precursor through a hydrothermal treatment method. The synthetic process was toxic chemical-free, convenient, and environmentally friendly. To find the optimized synthetic conditions, the temperature, heating time duration, and carbon precursor weight were evaluated. The prepared carbon dots were characterized by UV light, transmission electron microscopy, Raman, Fourier transform infrared, UV–vis, and fluorescence spectroscopy. The resulting carbon dots exhibit stable fluorescence with a quantum yield of approximately 10.85%. The carbon dots emitted the broad fluorescence emission range between 410 and 540 nm by changing the excitation wavelength and were used for the detection of Fe3+ ions at the excitation of 380 nm. It is found that Fe3+ ions induced the fluorescence intensity quenching of the carbon dots stronger than other heavy metals and the Fe3+ ion detection can be achieved within 3 min. Spectroscopic data showed that the obtained carbon dots can detect Fe3+ ions within the wide concentration range of 30–600 μM with 9.55 μM detection limit.

Carbon dots (CDs) are a new cluster of carbon atoms with particle size less than 10 nm. CDs also exhibit interesting fluorescence (FL) properties. CDs are attractive because of their fascinating characteristics including low toxicity, good water solubility, and tremendous biocompatibility. Recently, CDs have been investigated as biosensors for numerous target analytes. Meanwhile, the utilization of cheap and renewable natural resources not only fulfills the pressing requirement for the large-scale synthesis of CDs but also encourages the establishment of sustainable applications. The preparation of CDs using natural resources, i.e., plants, offers several advantages as it is inexpensive, eco-friendly, and highly available in the surroundings. Plant parts are readily available natural resources as the starting materials to produce CDs with different characteristics and attractive applications. Several review articles are now available covering the synthesis, properties, and applications of CDs. However, there is no specific and focused review literature discussing plant part-derived CDs for biosensing applications. To handle this gap, we provide a review of the progress of CDs derived from various plant parts with their synthesis methods, optical properties, and biosensing applications in the last five years. We highlight the synthesis methods and then give an overview of their optical properties and applications as biosensors for various biomolecules and molecules in biological samples. Finally, we discuss some future perspectives for plant part-derived CDs for better material development and applications.

In this study, we integrated zinc oxide nanomaterials, which possess a high surface-to-volume ratio and take part in specific interactions with organic functional groups, into infrared sensing devices to improve both the sensitivity and selectivity of the detection of volatile organic compounds (VOCs). An annealing method was developed to modify ZnO nanoparticles directly onto the surface of an IR internal reflection element. The ZnO nanoparticles produced this way are spherical (diameters, approximately 20 nm). When this modified sensing element was used to detect VOCs, intense IR signals for compounds bearing polar functional groups were observed. The conditions for preparing the ZnO nanoparticles for IR sensing of VOCs were optimized by varying such factors as the volume of the coating zinc solution, the calcination temperature, and calcination time. After mapping the IR signals obtained with respect to these factors, the optimal IR signal from this modified IR sensing element occurred when using 100 microL of zinc solution and performing the calcination at 400 degrees C for at least 30 min. VOCs having different functional groups were used to characterize the behavior of the ZnO-modified sensing element; our results indicate that the selectivity of this device favors polar compounds. Based on detection of several polar VOCs, the results indicate that quantitative analysis is possible when using the ZnO nanoparticle-modified sensor; in some cases, the detection limit was below an injected sample volume of 0.5 nL (approximately 2.2 ppm), with a linear regression coefficient (R2) above 0.99 when up to 0.3 microL of sample (approximately 1400 ppm) was injected to a 100 mL of sample cell.

In this study, a new application method for SERS named "reversed reporting agent" method is proposed for selective detection of biomolecules with a thiol group. In this method reporting agents such as rhodamine 6G (R6G) are capped on surfaces of silver colloidal nanoparticles. Analytes having a thiol group will replace the positions of reporting agents due to the strong interactions between silver and the thiol group, and then the SERS intensity of the reporting agents will be reduced. By monitoring the difference in the SERS intensity of reporting agents before and after the replacement of analytes, the concentration of the analytes can be estimated. To demonstrate the feasibility of this proposed method, glutathione was used as the analyte and R6G, crystal violet (CV), and thiacyanine (TC) were employed as the reporting agents, and factors that affect the capability of the determination of glutathione were investigated. The results indicate that R6G is the most suitable reporting agent, and the concentration of the reporting agents affects the sensitivity and selectivity of glutathione determination. Under the optimal conditions, the detection of glutathione can be finished within 20 minutes and the detection limit of ca. 1 microM can be achieved. Furthermore, the proposed method holds specific selectivity towards glutathione, which means it has possible practical applications.