We report a one-step route for the synthesis of Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 particles
in nanometer dimensions, with controllable shell thickness. This scalable procedure leads to stable and
freely dispersible particles, and bulk nanocomposite materials have been made this way. The procedure
leads to particles of various morphologies, with a crystalline core in the size range of 30−60 nm diameter
and an amorphous shell of ∼3 nm thickness in a typical synthesis. The core diameter and shell thickness
(in the range of 1−10 nm) can be varied, leading to different absorption maxima. The material has been
characterized with microscopic, diffraction, and spectroscopic techniques. The metal particle growth occurs
by the carbamic acid reduction route followed by hydrolysis of the metal oxide precursor, resulting in the
oxide cover. The particles could be precipitated and redispersed. The shell, upon thermal treatment, gets
converted to crystalline oxides. Cyclic voltammetric studies confirm the core−shell structure. The E
1/2
value is 0.250 V (ΔE ≈ 180 mV) for the quasi-reversible Ag
m
/Ag
m
+ couple and 0.320 V (ΔE ≈ 100 mV) for
the Au
n
/Au
n
+ couple for Ag and Au particles, respectively. Adsorption on the oxide surface blocks electron
transfer partially. Nonlinear optical measurements in solutions show that these materials are strong
optical limiters with a high laser damage threshold.
The antibacterial drug ciprofloxacin (cfH) has been used to protect gold nanoparticles of two different mean diameters, 4 and 20 nm. The protection is complete with about 65 and 585 cfH molecules covering 4 and 15 nm particles, respectively. The nature of binding has been investigated by several analytical techniques. The nitrogen atom of the NH moiety of piperazine group binds on the gold surface, as revealed by voltammetric and spectroscopic studies. The cfH-adsorbed particles are stable in the dry state as well as at room temperature, and as a result, redispersion is possible. The rate of release of the drug molecule from the nanoparticles is more in the basic medium than in pure water, and the kinetics depend on the size of the particle; faster desorption is seen in smaller particles. The bound cfH is fluorescent, and this property could be used in biological investigations. This study shows that metal nanoparticles could be useful carriers for cfH and fluoroquinolone molecules. Most of the bound molecules could be released over an extended period of time.
Bioconjugates of the hemoproteins, myoglobin, and hemoglobin have been synthesized by their adsorption on spherical gold and silver nanoparticles and gold nanorods. The adsorption of hemoproteins on the nanoparticle surface was confirmed by their molecular ion signatures in matrix assisted laser desorption ionization mass spectrometry and specific Raman features of the prosthetic heme b units. High-resolution transmission electron microscopy (HRTEM) and UV-visible spectroscopy showed that the particles retain their morphology and show aggregation only in the case of silver. The binding of azide ion to the Fe(III) center of the prosthetic heme b moiety caused a red shift of the Soret band, both in the case of the bioconjugates and in free hemoproteins. This was further confirmed by the characteristic signature at 2050 cm-1 in the Fourier-transform infrared spectra, which corresponds to the asymmetric stretching of the Fe(III) bound azide. The retention of the chemical behavior of the prosthetic heme group after adsorption on the nanoparticle is interesting due to its implications in nanoparticle supported enzyme catalysis. The absence of morphology changes after the reaction of bioconjugates with azide ion observed in HRTEM studies implies the stability of nanoparticles under the reaction conditions. All these studies indicate the retention of protein structure after adsorption on the nanoparticle surface.
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