Abstract
The present work reports a rapid, simple and efficient one‐step synthesis and detailed characterisation of stable aqueous colloids of gold nanoparticles (AuNPs) coated with unmodified poly(ethylene)glycol (PEG) molecules of different molecular weights and surface charges. By mixing and heating aqueous solutions of PEG with variable molecular chain and gold(III) chloride hydrate (HAuCl4) in the presence of NaOH, we have successfully produced uniform colloidal 5 nm PEG coated AuNPs of spherical shape with tunable surface charge and an average diameter of 30 nm within a few minutes. It has been found out that PEGylated AuNPs provide optical enhancement of the characteristic vibrational bands of PEG molecules attached to the gold surface when they are excited with both visible (532 nm) and NIR (785 nm) laser lines. The surface enhanced Raman scattering (SERS) signal does not depend on the length of the PEG molecular chain enveloping the AuNPs, and the stability of the colloid is not affected by the addition of concentrated salt solution (0.1 M NaCl), thus suggesting their potential use for in vitro and in vivo applications. Moreover, by gradually changing the chain length of the biopolymer, we were able to control nanoparticles’ surface charge from −28 to −2 mV, without any modification of the Raman enhancement properties and of the colloidal stability.
Highlights
The ability of tuning the physicochemical properties of different classes of metallic and/or semiconductor nanoparticles by controlling their size in the nanoscale put them in the limelight of the modern biomedical research
The morphology of all the produced gold colloids has been characterized by using ultraviolet-visible spectroscopy (UV-VIS) spectroscopy and transmission electron microscopy (TEM)
The values of the absorption maximum are a characteristic for surface plasmon resonance (SPR) absorbance of the spherical aqueous colloids of gold nanoparticles (AuNPs) induced by the interaction between the incoming light and the conduction electrons of the spherical metallic nanoparticle [15]
Summary
The ability of tuning the physicochemical properties of different classes of metallic and/or semiconductor nanoparticles by controlling their size in the nanoscale put them in the limelight of the modern biomedical research. Several classes of nanoparticles: quantum dots, polymer nanoparticles [1], dendrimers [2], liposomes [3], nanotubes [4], and nanorods [5], have been synthesized and characterized Owing to their unique properties (enhancement of Raman signal, size-tunable surface plasmon resonance, ease of preparation and of surface functionalization), gold nanoparticles (AuNPs) have become one of the most interesting candidates for modern biomedical applications such as biosensing, targeted drug delivery and photothermal therapy [6, 7]. Zeta potential (mV) −28,2 −25,5 −16,2 −12,1 −7,72 −2,16 −1,94 developed by Brust et al more than 40 years ago allowed the synthesis of smaller gold nanoparticles and their subsequent functionalization with thiolated molecules [9] The success of this method is mainly due to the possibility of incorporating chemical functionalities on the nanoparticle surface as novel biomedical applications require nanoparticles functionalized with different species of biomolecules. The as-prepared colloids have been investigated by UV-VIS absorption spectroscopy, transmission electron microscopy (TEM), zeta potential measurements, and surface-enhanced Raman spectroscopy
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