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Fig. 6 a Optical spectra for solutions containing citrate-capped gold nanoparticles with various
copper (II) ion concentrations; surface plasmon band peak is gradually redshifting upon addition of
copper (II) ions; b calibration curve based on plasmon band maximum shifts, from optical spectra;
c photograph showing aqueous solutions of gold nanoparticles functionalized with citrate; the
noticeable gradual color changes recorded upon addition of copper (II) ions is due to nanoparticle
aggregation (coupling of the localized plasmon bands). (Color figure online)