Article pubs.acs.org/ac
Raman Analysis of Dilute Aqueous Samples by Localized Evaporation of Submicroliter Droplets on the Tips of Superhydrophobic Copper Wires Melody Cheung, Wendy W. Y. Lee, John N. McCracken, Iain A. Larmour, Steven Brennan, and Steven E. J. Bell* Innovative Molecular Materials Group, School of Chemistry and Chemical Engineering, Queen’s University, David Keir Building, Stranmillis Road, Belfast, United Kingdom, BT9 5AG S Supporting Information *
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.
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meaning the signal will vary dramatically over the area of the dried deposit. The best way to circumvent the coffee ring effect is to use a superhydrophobic (SHP) surface so that the deposited water droplet shrinks radially,4 leaving a small solid deposit which can then be probed. The main disadvantage of this approach is that, while the deposit is indeed localized (for SHP pillar arrays the deposit has been shown to be suspended between 3 pillars), the precise location of the deposit is not fixed, so it must be located by manual searching.5,6 What is required is a method in which the drying droplet is directed to a known point on the surface so that the location of the deposit will be fixed. This can be achieved by preparing complex micropatterned structures on silicon substrates to produce a hydrophobicity gradient. Drying droplets of gold colloids on these surfaces has been shown to create deposits ∼100 μm in diameter to which further sample droplets could be added to
ecent years have seen a rise in the interest and practical use of Raman spectroscopy as an analytical technique. However, one of the main problems associated with Raman spectroscopy is the low scattering probability which leads to low signal intensities and means that normal Raman scattering can really only be applied to solid materials which contain a significant proportion of the target of interest or concentrated solutions. Of course surface-enhanced methods can be used to increase the scattering cross sections of target molecules which are adsorbed onto the surface of enhancing substrates, but this can only be applied to a subset of all possible targets since many compounds simply do not adsorb.1−3 An alternative approach to Raman analysis of dilute samples is to concentrate them by evaporating off some or all of the solvent. This has considerable potential since it is a very general method and the solid residue needs only to be the diameter of the probing laser in order to be effectively Raman probed. The simplest method is to dry a droplet directly onto a simple planar substrate such as a glass slide or a polished metal plate with a thin Teflon coating. This typically leads to uneven sample deposition due to a combination of sample spreading and the “coffee ring” effect, © 2016 American Chemical Society
Received: February 10, 2016 Accepted: March 31, 2016 Published: March 31, 2016 4541
DOI: 10.1021/acs.analchem.6b00563 Anal. Chem. 2016, 88, 4541−4547
Analytical Chemistry
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give surface-enhanced Raman spectroscopy (SERS) signals.7 Here, we describe a much more straightforward method8 for preparing substrates which localize the deposits formed from drying droplets using copper wires and room temperature wet chemical processing. The most important requirement for the substrates is that they are SHP, but we have previously reported a very simple and most effective method of preparing SHP surfaces on copper metal using electroless galvanic deposition of Ag followed by surface modification with a polyfluoro thiol (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, HDFT).8,9 This method takes a few minutes and requires no specialist equipment. The coated wires are completely coated in a SHP material and can then be cut to expose a hydrophilic tip onto which the analyte can be deposited. Drying of the droplet leaves a solid residue which is automatically located on the exposed cut surface. Here, the method was tested with solutions of melamine as a conventional organic target molecule and with sucrose and glucose which give relatively poor detection limits even with SERS.10−12
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Article
RESULTS AND DISCUSSION
The preparation method is extremely straightforward since it is based on our method for electroless deposition of a microtextured Ag coating followed by surface modification with a low surface energy self-assembled monolayer (SAM) of a polyfluorothiol. This is known to give surfaces with very high equilibrium water contact angles ca. 170°. The main challenge is how to create a hydrophilic spot without mechanically or chemically damaging the rest of the coating. We have explored the possibility of creating flat SHP surfaces and then either removing small (
