Increasing hotspots density for high-sensitivity SERS detection by assembling array of Ag nanocubes.

Trace analysis has great promise in the fields of disease diagnosis and environment protection. Surface-enhanced Raman scattering (SERS) has wide range of utilization due to its reliable fingerprint detection. However, the sensitivity of SERS still needs to be enhanced. Raman scattering of target molecules around hotspots, the area with extremely strong electromagnetic field, can be highly amplified. Therefore, to increase the density of hotspots is one of the major approaches for enhancing the detection sensitivity of target molecules. In this paper, an ordered array of Ag nanocubes was assembled on a thiol modified silicon substrate as a SERS substrate, which provided high-density hotspots. The detection sensitivity is demonstrated by the limit of detection, which is down to 10-6 nM with Rhodamine 6G as probe molecule. The wide linear range (10-7-10-13 M) and low relative standard deviation (<6.48%) indicate the good reproducibility of the substrate. Furthermore, the substrate can be used for the detection of dye molecules in lake water. This method provides an approach for increasing hotspots of SERS substrate, which could be a promising method to achieve good reproducibility and high sensitivity.

Laser desorption/ionization on nanostructured silicon: morphology matters.

The surface morphology of the silicon nanostructure plays a crucial role in the laser desorption/ionization (LDI) process. Understanding the correlation between the surface morphology and LDI performance is the foundation for creating silicon substrates with high LDI efficiency. Most of the present studies focus only on the structural parameters (such as porosity, depth, total surface area, dimension, etc.) of a single structure, but their effects on LDI efficiency vary with the types of silicon structures. Herein, two representative types of silicon nanostructures, porous silicon (PSi) and thorny silicon (TSi), were created to address this issue. The results indicate that the PSi substrate can generate a stronger heat effect and is beneficial to desorption; the TSi substrate can facilitate electron transfer and is favorable to ionization. Subsequently, the assertion was further confirmed by simultaneously detecting a dozen of standard samples and a real sample on both the TSi and PSi substrates, in which PSi can significantly enhance the detection signals of organic salts, whereas the TSi substrate can greatly increase the LDI efficiencies of neutral analytes. This finding provides a foundation for improving the LDI performance by tailoring silicon nanostructures, which is helpful for designing and creating substrates with high LDI performance.

Silver-nanoparticle-grafted silicon nanocones for reproducible Raman detection of trace contaminants in complex liquid environments.

Super-hydrophobic delivery (SHD) is an efficient approach to enrich trace analytes into hot spot regions for ultrasensitive surface-enhanced Raman scattering (SERS) detection. In this article, we propose an efficient and simple method to prepare a highly-uniform SHD-SERS platform of high performance in trace detection, named as "silver-nanoparticle-grafted silicon nanocones" (termed AgNPs/SiNC) platform. It is fabricated via droplet-confined electroless deposition on the super-hydrophobic SiNC array. The AgNPs/SiNC platform allows trace analytes enriched into hot spots formed by AgNPs, leading to an excellent reproducibility and sensitivity. The relative standard deviation (RSD) for detecting R6G (10-7 M) is down to 4.70% and the lowest detection concentration for R6G is 10-14 M. Moreover, various contaminants in complex liquid environments, such as, crystal violet (10-9 M) in lake water, melamine (10-7 M) in liquid milk and methyl parathion (10-7 M) in tap water, can be detected using the SERS platform. This result demonstrates the great potential of the AgNPs/SiNC platform in the fields of food safety and environmental monitoring.

Eliminating sweet spot in MALDI-MS with hydrophobic ordered structure as target for quantifying biomolecules.

In matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), the analyte is usually distributed unevenly throughout the sample spot. The area with aggregated analyte molecules contributing abundant signal, is termed as "sweet spot", which results in poor detection reproducibility and makes it impossible to quantify analytes without internal standards. We proposed a strategy to eliminate sweet spot in MALDI-MS by using a hydrophobic ordered structure as target. The target is fabricated by creating a hydrophobic silicon nanopillar array and subsequently decorating it uniformly with poly(methyl methacrylate) nanodots for capturing analytes. The sweet spot is eliminated by distributing analyte molecules uniformly on this target, and then result in a uniform MS image, which demonstrates an ideal reproducibility. Finally, with the target assisted MALDI-MS as biosensor was suitable to analyze practical sample such as bacitracin A in milk. Horse heart myoglobin and, angiotensin III molecules can be quantified without internal standard using α-cyano-4-hydroxycinnamic acid as matrix. This biosensor presented good linearity, high salts tolerance and high signal-to-noise ratio (up to 271.8), even the 1 mol/L salt concentration. This strategy could provide an alternative for improving the performance of MALDI-MS.

Carbon nanoparticles derived from carbon soot as a matrix for SALDI-MS analysis.

Carbon nanoparticles (NPs) from the incomplete combustion of a candle were used as matrix for surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The washed carbon soot NPs (WCS NPs, ~48 nm) exhibit higher laser desorption/ionization efficiency and less background compared with other common metal and carbon matrices. WCS NPs present good reproducibility and high sensitivity in analyzing a wide range of molecules in both positive and negative ionization mode in SALDI-MS. The detection limit of glucose is 1 pmol with WCS NPs as matrix. WCS NPs can be used to quantitatively determine urine glucose, visualize latent fingerprint and image it with SALDI-MS. The UV absorption of WCS NPs and MS spectra analyzed with WCS NPs matrix remain the same after 10 months storage, indicating the good stability of WCS NPs as matrix. Graphical abstractSchematic representation of carbon nanoparticles derived from carbon soot and its application as matrix in SALDI-MS.

Enriching analyte molecules on tips of superhydrophobic gold nanocones for trace detection with SALDI-MS.

The sensitivity of surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) analysis depends on the efficiency of desorption and ionization of analyte molecules, which is usually limited by the low utilization efficiency of laser energy. Herein we demonstrate an efficient method to increase energy utilization efficiency for improving the efficiency of desorption and ionization of analyte molecules in SALDI-MS analysis. To increase the utilization efficiency of energy, a superhydrophobic gold film covered silicon nanocone array is fabricated and used as SALDI substrate. The nanocone array increases the absorption up to 99.65% at the wavelength of 355 nm, which is applied for SALDI-MS detection. The superhydrophobicity promotes the analyte molecules concentrated on the tips of nanocones where photon energy is confined, therefore, more energy can be provided for desorption and ionization of analytes. The energy efficiency is increased by using this substrate. The sensitivity of SALDI-MS analysis is greatly improved. For example, 100 amol/µL of rhodamine 6G, 100 fmol/µL of polyethyleneglycol, 100 ymol/µL of glutathione and 100 ymol/µL arginine still can be analyzed. The lake water containing malachite green was used as the real sample. The regression equation (Log I = 0.39 Log C + 6.58, R2 = 0.9811) was obtained when the concentration of analyte was in the range from 10-4 mol/L to 10-8 mol/L. Therefore, the calculated LOD and LOQ are 1.35 × 10-14 mol/L and 1.35 × 10-7 mol/L, respectively. In addition, the lower relative standard deviation (0.7%, n = 10), proper recovery (113% and 91%), and low matrix effect (-1.1% and -1.1%) all demonstrate the great potential of the designed substrate in practical analysis.