Synthesis of SERS active nanoparticles for detection of biomolecules

Surface Enhanced Raman Scattering (SERS) can be used to detect specific DNA sequences by methods based on hybridisation of oligonucleotide functionalized nanoparticles to complementary DNA sequences. The problem, which has to be overcome to use this technique is that DNA is not strongly SERS active. This is due to the lack of a visible chromophore and presence of the highly negatively charged phosphate backbone, which prevents the electrostatic interaction with the metal surface necessary for the enhancement. To obtain SERS active DNA a label containing a surface seeking group, to allow adsorption of DNA on a metal surface, and a chromophore has to be attached to the DNA strand. Here we report the synthesis of three linkers containing a Raman tag [the following fluorophores were used for this purpose due to the fluorescence quenching ability of metallic nanoparticles: fluorescein, 6-aminofluorescein and tetramethylrhodamine (TAMRA)], surface complexing group (cyclic disulphide—thioctic acid) and a chemical functionality for attachment of DNA (carboxyl group). Each of the linkers also contain poly(ethylene glycol) (PEG) (3 mer), which reduces non-specific adsorption of molecules to the surface of the nanoparticles and provides colloidal stability. The synthesized linkers were used to functionalize gold citrate (18 and 50nm), silver citrate (40nm) and silver EDTA (35nm) nanoparticles. All of the conjugates exhibit high stability, gave good SERS responses at laser excitation frequencies of 514 and 633nm and could be conjugated to amino-modified oligonucleotides in the presence of the commonly used (N-(3-dimethylaminopropyl)-N′-ethyl carbodiimide hydrochloride—EDC·HCl with N-hydroxysulfosuccinimide or 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride—DMT MM, which has not been used for bioconjugate preparation previously. This approach is less time consuming and less expensive than previously used protocols and does not require the formation of a mixed layer of oligonucleotides and Raman reporter on the metal surface. Additionally the presence of a reactive functionality within the linker structure makes it possible to conjugate the linker to other biomolecules of interest such as proteins.