Influence of Ag Photodeposition Conditions over SERS Intensity of Ag/ZnO Microspheres for Nanomolar Detection of Methylene Blue

Surface enhanced Raman spectroscopy (SERS) is considered a versatile and multifunctional technique with the ability to detect molecules of different species at very low molar concentration. In this work, hierarchical ZnO microspheres (ZnO MSs) and Ag/ZnO MSs were fabricated and decorated by hydrothermal and photodeposition methods, respectively. For Ag deposition, precursor molar concentration (1.9 and 9.8 mM) and UV irradiation time (5, 15, and 30 min) were evaluated by SEM, TEM, X-ray diffraction and Raman spectroscopy. X-ray diffraction showed a peak at 37.9° corresponding to the (111) plane of Ag, whose intensity increases as precursor concentration and UV irradiation time increases. SEM images confirmed the formation of ZnO MSs (from 2.5 to 4.5 µm) building by radially aligned two-dimensional ZnO nanosheets with thicknesses below 30 nm. The Raman spectra of Ag/ZnO MSs exhibited a vibration mode at 486 cm−1 which can be directly associated to Ag deposition on ZnO MSs surface. The performance of SERS substrate was evaluated using rhodamine 6G. The SERS substrate grown at 9.8 mM during 30 min showed the best SERS activity and the ability to detect methylene blue at 10−9 M.

Versatile Silver Nanoparticles-Based SERS Substrate with High Sensitivity and Stability

Surface-enhanced Raman scattering has developed into a mature analytical technique useful in various applications; however, the reproducible fabrication of a portable SERS substrate with high sensitivity and good uniformity is still an ongoing pursuit. Reported herein is a rapid fabrication method of an inexpensive SERS substrate that enables sub-nanomolar detection of molecular analytes. The SERS substrate is obtained by application of silver nanoparticles (Ag NPs)-based ink in precisely design patterns with the aid of an in-house assembled printer equipped with a user-fillable pen. Finite-difference time-domain (FDTD) simulations show a 155-times Ag NP electric field enhancement for Ag nanoparticle pairs with particle spacing of 2 nm. By comparing the SERS performance of SERS substrate made with different support matrices and fabrication methods, the PET-printed substrate shows optimal performance, with an estimated sensitivity enhancement factor of 107. The quantitative analysis of rhodamine 6G absorbed on optimized SERS substrate exhibits a good linear relationship, with a correlation coefficient (R2) of 0.9998, between the SERS intensity at 610 cm−1 and the concentration in the range of 0.1 nM—1μM. The practical low limit detection of R6G is 10 pM. The optimized SERS substrates show good stability (at least one month) and have been effectively tested in the detection of cancer drugs, including doxorubicin and metvan.

Efficient Sub-1 Minute Analysis of Selected Biomarker Catecholamines by Core-Shell Hydrophilic Interaction Liquid Chromatography (HILIC) with Nanomolar Detection at a Boron-Doped Diamond (BDD) Electrode

A rapid, sensitive method for the separation of catecholamine biomarkers (CAs), of importance in traumatic brain injury (TBI) and in Parkinson’s disease (PD), has been successfully developed using hydrophilic interaction liquid chromatography (HILIC). Dopamine (DA), epinephrine (EPI), and norepinephrine (NE) are known to be three to fivefold elevated above normal in traumatic brain injury (TBI) patients. HILIC facilitates the rapid and efficient separation of these polar biomarkers, which can be poorly retained by reversed-phase liquid chromatography (RPLC), while electrochemical detection (ECD) at the boron-doped diamond (BDD) electrode provides enhanced nanomolar detection. Three HILIC columns were compared, namely the superficially porous (core-shell) Z-HILIC column and the Z-cHILIC and Z-HILIC fully porous columns. The core-shell Z-HILIC showed the highest efficiency with a rapid separation within 60 s. The HILIC method utilizing the core-shell Z-HILIC column was initially optimized for the simultaneous analysis of DA, EPI, and NE using UV detection. The advantages of using the BDD electrode over UV detection were explored, and the improved limits of detection (LODs, S/N = 3) measured were 40, 50, and 50 nM for DA, EPI, and NE, respectively. Method validation is reported in terms of the linearity, repeatability, reproducibility, and LODs. Furthermore, the proposed method was successfully applied to the real sample analysis of urinary CAs following phenylboronic acid (PBA) solid phase extraction (SPE) pretreatment.