Abstract
It is well known that spontaneous drying of some fluid droplets on certain solid surfaces forms a “coffee ring” pattern. In this paper, we studied “coffee ring” formation for two kinds of Ag colloidal nanoparticles (borohydride-reduced (b.-r.) and hydroxylamine-reduced (h.-r.)) and its impact on surface-enhanced Raman scattering (SERS). Optical and scanning electron microscopies were used to observe the morphology of the dried rings as well. We used 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP) as a testing SERS molecular probe. The results showed that the structure of the edge rings of dried drops of Ag colloid/TMPyP systems was different for b.-r. and h.-r. nanoparticles. The inherent limitation of our approach is inhomogeneity in particle and “hot spots” distribution, SERS signal fluctuation, and consequently low spectral reproducibility. However, in the case of h.-r. nanoparticles, it formed a structure with highly enhancing sites (“hot spots”) providing enormous SERS signal of TMPyP. Higher sensitivity and the possibility of spectral mapping over the dried pattern are advantages in comparison with the measurements from colloidal suspension. Although our approach is not reliable for quantitative analytical SERS applications, it can serve as a simple, cheap, and fast prescan method, which can be easily implemented for preliminary SERS analysis.
Highlights
It is well known that a spontaneous drying of a fluid droplet on a solid surface at ordinary room temperature causes a fluid movement radially outward to the contact line in order to maintain its position
We propose that our approach can serve as a simple, cheap, and fast prescan method, which can be implemented for preliminary surface-enhanced Raman scattering (SERS) analysis
The results proved that dried drops of Ag colloid/TMPyP systems on glass slides formed compact rings containing nanoparticle aggregates
Summary
It is well known that a spontaneous drying of a fluid droplet on a solid surface at ordinary room temperature causes a fluid movement radially outward to the contact line in order to maintain its position. Au and Ag nanoparticles are frequently used as active substrates for surface-enhanced Raman scattering (SERS) [9]. SERS is a useful detection and analytical technique based on an enormous enhancement of Raman scattering (usually up to 106) from molecules adsorbed on a roughened metal surface (so-called “SERS-active substrate”) [9]. Drying by a “coffee-ring” effect promotes adsorption of the studied analytes to nanoparticle surface as well as aggregation and packing of nanoparticles in the ring at the edge of the droplet [10, 11]. This results in high SERS enhancement but low spectral reproducibility. Such approach is simple and was successfully used for SERS detection of lipids [11], proteins [12], and uric acid [13]
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