Abstract
Layers formed from single-stranded DNA on nanostructured plasmonic metals can be applied as “working elements” in surface–enhanced Raman scattering (SERS) sensors used to sensitively and accurately identify specific DNA fragments in various biological samples (for example, in samples of blood). Therefore, the proper formation of the desired DNA layers on SERS substrates is of great practical importance, and many research groups are working to improve the process in forming such structures. In this work, we propose two modifications of a standard method used for depositing DNA with an attached linking thiol moiety on certain SERS-active structures; the modifications yield DNA layers that generate a stronger SERS signal. We propose: (i) freezing the sample when forming DNA layers on the nanoparticles, and (ii) when forming DNA layers on SERS-active macroscopic silver substrates, using ω-substituted alkanethiols with very short alkane chains (such as cysteamine or mercaptopropionic acid) to backfill the empty spaces on the metal surface unoccupied by DNA. When 6-mercapto-1-hexanol is used to fill the unoccupied places on a silver surface (as in experiments on standard gold substrates), a quick detachment of chemisorbed DNA from the silver surface is observed. Whereas, using ω-substituted alkanethiols with a shorter alkane chain makes it possible to easily form mixed DNA/backfilling thiol monolayers. Probably, the significantly lower desorption rate of the thiolated DNA induced by alkanethiols with shorter chains is due to the lower stabilization energy in monolayers formed from such compounds.
Highlights
In recent years, there has been much research activity in developing new techniques enabling genetic disorders to be diagnosed through the identification of DNA chains having a given sequence in various biological samples
As mentioned in the introduction, we decided to analyze how certain modifications of the method for depositing thiolated DNA on plasmonic nanoparticles might affect the intensity and repeatability of the surface-enhanced Raman scattering (SERS) spectra recorded, and how the chemisorption of various alkanethiols used to fill those places on the metal surface unoccupied by DNA could affect the structure of the obtained layer
For both the labeled and label-free DNA chains, the SERS spectra obtained for the samples prepared using a freezing step were more intense than the spectra of the samples prepared by the standard method of incubation at room temperature
Summary
There has been much research activity in developing new techniques enabling genetic disorders to be diagnosed through the identification of DNA chains having a given sequence in various biological samples (for example, in blood). The formation of DNA layers on SERS-active plasmonic substrates is being studied and developed by many groups around the world In this contribution, we propose two modifications of this process leading to layers that yield a stronger SERS signal of DNA: (1) freezing the sample during the adsorption of DNA with an attached thiol linking moiety on plasmonic nanoparticles [29,30,31], and (2) in the case of the formation of DNA layers on SERS-active macroscopic silver substrates (silver substrates usually generate a stronger SERS enhancement factor than the gold substrates commonly used [32]), using a compound other than 6-mercapto-1-hexanol (normally used on standard gold substrates) to fill the places on the metal surface unoccupied by DNA [18,33,34]
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