Sensitive
Cylindrical SERS Substrate Array for Rapid
Microanalysis of Nucleobases
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Abstract
In
this work, a cylindrical-substrate array for surface-enhanced
Raman scattering (SERS) measurements was developed to enable analysis
of nucleobases in a few microliters of liquid. To eliminate uncertainties
associated with SERS detection of aqueous samples, a new type of cylindrical
SERS substrate was designed to confine the aqueous sample at the tip
of the SERS probe. Poly(methyl methacrylate) (PMMA) optical fibers
in a series of different diameters were used as the basic substrate.
A solution of poly(vinylidene fluoride)/dimethylformamide (PVDF/DMF)
was used to coat the tip of each fiber to increase the surface roughness
and facilitate adsorption of silver nanoparticles (AgNPs) for enhancing
Raman signals. A chemical reduction method was used to form AgNPs
in and on the PVDF coating layer. The reagents and reaction conditions
were systematically examined with the aim of estimating the optimum
parameters. Unlike the spreading of aqueous sample on most SERS substrates,
particularly flat ones, the new SERS substrates showed enough hydrophobicity
to restrict aqueous sample to the tip area, thus enabling quantitative
analysis. The required volume of sample could be as low as 1 μL
with no need for a drying step in the procedure. By aligning the cylindrical
SERS substrates into a solid holder, an array of cylindrical substrates
was produced for mass analysis of aqueous samples. This new substrate
improves both reproducibility and sensitivity for detection in aqueous
samples. The enhancement factor approaches 7 orders in magnitude with
a relative standard error close to 8%. Using the optimized conditions,
nucleobases of adenine, cytosine, thymine, and uracil could be detected
with limits approaching a few hundreds nanomolar in only a few microliters
of solution