Processing and Characterization of Gold Nanoparticles for Use in Plasmon Probe Spectroscopy and M icroscopy of Biosystem s YU CHEN, a,b J ON A. PREECE, c AND RICHARD E. PALMER b a Department of Physics, University of Strathclyde, Glasgow, United Kingdom b Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom c School of Chemistry, University of Birmingham, Birmingham, United Kingdom Noble metal nanoparticles have great potential for applications in biochemical sensing and bio- logical imaging because of their unique optical properties originating from the excitation of local surface plasmon resonances. We investigated gold nanoparticles with controlled size, shape, and passivating agents, along with a new process of guided self-assembly to create two-dimensional nanostructures from such nanoparticles. Key words: nanoparticles; nanoclusters; synthesis; processing; surface plasmon Introduction Noble metal nanoparticles have great potential for applications in biochemical sensing and biolog- ical imaging because of their unique optical prop- erties originating from the excitation of local sur- face plasmon resonances (LSPRs). 1–3 The highly confined local electric field enhancement that accom- panies the excitation of LSPRs has been used in a vari- ety of near-field enhanced spectroscopy and imaging modes, from near-field scanning optical microscopy to surface-enhanced Raman spectroscopy. 4–6 Moreover, enhancement of fluorescence by using near-field ef- fects has been achieved in single-molecule experiments and planar dye layers with adsorbed nanoparticles. 7–9 Among noble metal particles, gold nanoparticles have attracted intensive interest because they are easily pre- pared, have low toxicity, and can be readily attached to molecules of biological interest. 10 The surface plasmon resonance is a coherent oscil- lation of the surface conduction electrons excited by electromagnetic radiation. It is sensitive to local dielec- tric environment. 11–13 Typically, LSPR devices sense changes in the local environment through a resonance wavelength shift. Apart from the environmental effect, Address for correspondence: Yu Chen, Department of Physics, Uni- versity of Strathclyde, John Anderson Bldg., 107 Rottenrow, Glasgow G4 0NG, UK. Voice: +0044 141 548 3087; fax: +0044 141 552 2891. y.chen@strath.ac.uk the LSPR of nanoparticles is dramatically affected by their size, shape, and surface modifications. Therefore, one can tune the LSPR wavelength throughout the visible, near-infrared, and infrared region of the elec- tromagnetic spectrum by careful control over the syn- thetic process to vary the nanoparticle shape, size, and encapsulation. We investigated gold nanoparticles with controlled size, shape, and passivating agents, along with a new process of guided self-assembly to create two-dimensional nanostructures from such nanoparti- cles. Synthesis of Gold Nanoparticles Gold colloidal nanoparticles were synthesized with different passivating ligands, including didecyl sulfides, citrate, and magnesium oleate molecules. Gold nanoparticles passivated with didecyl sulfide (H 21 C 10 SC 10 H 21 ) were synthesized through the boro- hydride reduction of HAuCl 4 . 14 Ultraviolet (UV) to visible spectra of gold colloids stabilized by didecyl sul- fide showed broad absorption bands around 520nm, corresponding to nanoparticles in the size range of 5– 6 nm. Analysis of the transmission electron microscopy (TEM) photomicrographs of the nanoparticles was in agreement with these results (FIG. 1A). From the analy- sis of the images, the average diameter of gold particles passivated with didecyl sulfide was 5.3 ± 0.8 nm. High-resolution electron energy loss spectroscopy (HREELS) is a highly surface-sensitive technique and Ann. N.Y. Acad. Sci. 1130: 201–206 (2008). C  2008 New York Academy of Sciences. doi: 10.1196/annals.1430.051 201