Since its discovery in 1974, surface-enhanced Raman spectroscopy (SERS) has become a widely used analytical technique offering many advantages over other techniques such as FT-IR spectroscopy, UV-visible-near infrared (UV–vis-NIR) absorption, X-ray photoelectron spectroscopy, mass spectrometry, etc. In the last few years, SERS became very popular in life science applications due to a great amount of information extracted from complex biological environments such as tissues, cell cultures, and biological fluids [1
]. Although numerous surfaces have been successfully tested as SERS-active substrates (Ag, Au, Cu, Na, Li, Pd, Pt) [4
], the best results for biomedical applications have been observed in the case of silver and gold nanoparticles [5
]. Compared with gold, silver offers two major advantages: the SERS enhancement factor is 10 to 100 times higher, and it can be excited from the UV to the infrared (IR) region, while gold is restricted to the IR due to the damping induced by interband transitions [6
] which have to be taken into account at the nanoscale.
The preparation of silver nanoparticles (AgNPs) is commonly done by reducing the silver ions of a precursor in a solution, usually aqueous media, and preventing particle growth by utilizing stabilizing agents such as surfactants and polymers. In this line, efficient methods of AgNP synthesis have been developed, i.e., the chemical reduction of silver salt solution by a reducing agent such as citrate, NaBH4
, hydrazine, and hydroxylamine hydrochloride [7
]. Moreover, given the enormous potential of these nanoparticles in biomedical applications envisaged in the last few years, a more biological approach has been developed for AgNP synthesis by functionalizing them with various biomedical and pharmaceutical substances able to enhance their absorption into malign cells. For proper application in in vivo
experiments, these novel nanoparticles must overcome several challenging requirements such as biocompatibility, stability in physiological solutions, non-toxicity, and ability to traverse biological barriers. A general strategy employed by many research groups in fulfilling these requirements is based on coating the nanoparticles with different classes of biopolymers. Since polyethylene glycol (PEG) is one of the most versatile biopolymer, environmentally benign and already used in the pharmaceutical and biomedical industries, much of the research interest has been focused on developing new methods of PEGylation. The successful attachment of PEG molecules onto the nanoparticle surface has already been done by adding SH-modified PEG molecules on previously synthesized AgNPs [10
] or using PEG as both reducing and stabilizing agents without [11
] or within aqueous media [14
]. Although the already reported methods are successful, they have two major drawbacks: the time required for the complete formation of PEG-functionalized AgNPs can reach several hours, and the methodology is quite complex in most of the cases.
In this paper, we report a simple, green, effective, and extremely fast method in preparing stable, highly SERS-active, and biocompatible silver colloids by the reduction of silver nitrate with PEG 200 at alkaline pH in aqueous media. The addition of sodium hydroxide shifts the solution pH towards the alkaline environment, thus reducing the reaction time from several hours to a few seconds. Sequential studies certified that the use of unmodified PEG molecules as reducing agent allows the successful formation of AgNPs. The key element of our method is in the presence of additional -OH groups generated in the solution by sodium hydroxide, enhancing the speed of chemical reduction of silver ions. Astonishing is the fact that Ag+ can be steadily reduced to Ag0 in such mild conditions, and remarkable is the fact that direct and cleaner AgNPs have been synthesized in a few seconds without using any mediators in the process. The as-produced silver colloids have been characterized by UV–vis spectrometry, transmission electron microscopy (TEM), and SERS. The SERS activity of silver colloids was tested using various analytes and was compared with those given by both citrate- and hydroxylamine-reduced silver colloids.