Raman Analysis of Dilute Aqueous Samples by Localized Evaporation of Submicroliter Droplets on the Tips of Superhydrophobic Copper Wires

Abstract

Raman analysis of dilute aqueous solutions is normally prevented by their low signal levels. A very general method to increase the concentration to detectable levels is to evaporate droplets of the sample to dryness, creating solid deposits which are then Raman probed. Here, superhydrophobic (SHP) wires with hydrophilic tips have been used as supports for drying droplets, which have the advantage that the residue is automatically deposited at the tip. The SHP wires were readily prepared in minutes using electroless galvanic deposition of Ag onto copper wires followed by modification with a polyfluorothiol (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, HDFT). Cutting the coated wires with a scalpel revealed hydrophilic tips which could support droplets whose maximum size was determined by the wire diameter. Typically, 230 μm wires were used to support 0.6 μL droplets. Evaporation of dilute melamine droplets gave solid deposits which could be observed by scanning electron microscopy (SEM) and Raman spectroscopy. The limit of detection for melamine using a two stage evaporation procedure was 1 × 10(-6) mol dm(-3). The physical appearance of dried droplets of sucrose and glucose showed that the samples retained significant amounts of water, even under high vacuum. Nonetheless, the Raman detection limits of sucrose and glucose were 5 × 10(-4) and 2.5 × 10(-3) mol dm(-3), respectively, which is similar to the sensitivity reported for surface-enhanced Raman spectroscopy (SERS) detection of glucose. It was also possible to quantify the two sugars in mixtures at concentrations which were similar to those found in human blood through multivariate analysis.

Active and Ultrasensitive Chemical and Biosensing through Optothermally Generated Microbubble

“…However, through the natural evaporation of a sessile droplet, nanoparticles will be deposited at the edge of the droplet, not in the center. [91][92][93][94][95] In order to prove this concept, a sessile droplet of gold nanosphere solution of 276 nL volume was injected on the same gold-coated glass coverslip substrate and then let it evaporate naturally. The primary radial flow inside a droplet induced by the natural evaporation of a droplet 96 is schematically shown in Figure 4.5 (d).…”

A Simple SERS-Based Trace Sensing Platform Enabled by AuNPs-Analyte/AuNPs Double-Decker Structure on Wax-Coated Hydrophobic Surface

“…The evaporation process created a higher number of “hot spots” in confined area and better uniformity of nanomaterials distribution. Accordingly, many efforts have been made to produce and employ hydrophobic SERS substrates for enhanced SERS sensing (Li et al, 2014 ; Wallace et al, 2014 ; Cheung et al, 2016 ; Jayram et al, 2016 ). However, a simple method to fabricate hydrophobic substrate is still highly demanded.…”

SERS of meso-droplets supported on superhydrophobic wires allows exquisitely sensitive detection of dipicolinic acid, an anthrax biomarker, considerably below the infective dose

“…Previous work with the SHP supports 18 showed that drying down aqueous melamine or sugar droplets allowed an increased localised concentration of the analyte onto the probe area which in turn gave maximum Raman signals. The same approach can usually be applied to SERS analysis, where maximum signals can be obtained when the solvent in the test sample evaporates to give a higher concentration of the target analyte adsorbed onto the enhancing metal nanoparticles.…”

“…The main objective of this work is to reduce the total mass of DPA which can be detected by combining the sensitivity observed in previous SERS studies with the ability to manipulate and probe small (0.2 mL) sample volumes which is created using superhydrophobic (SHP) materials for sample handling. This work is a natural progression from our previous work where small droplets of melamine or sugar were dried down to create solid deposits which were probed using Raman spectroscopy rather than SERS 18 and other published studies in which small droplets of colloid were evaporated to dryness on SHP surfaces to leave solid deposits suitable for SERS. [19][20][21] However, the current approach is different from these studies since here the samples are manipulated and then analysed as meso-droplets (ca.…”

“…We have previously shown that reducing the diameter to 100 mm decreases the maximum droplet diameter by a factor of 2, which suggests an order of magnitude decrease in sample volume should be possible. 18 However, here we used the larger droplets for experimental convenience because it allows physically robust supports and simple dispensing of the measured volumes.…”