In s ti t u te o f Ph y s ic s Pu b l is h in g Na n o t e c h n o l o g y L u cía B S c a ffa r d i1,2 and Jorge O T och o1,3

In s t it u t e of Ph y s ic s P u bl is h in g AbstractT h e m e a s u re m e n t o f o p tic a l e x tin c tio n is u se d to d e te rm in e th e siz e o f n e a rly sp h e ric a l g o ld n a n o p a rtic le s su s p e n d e d in so lu tio n , p ro d u c e d by a re v e rse m ic e lle s p ro c e s s. T h e c o n tra st b e tw e e n th e m a x im u m a n d th e m in im u m in th e e x tin c tio n sp e c tra a ro u n d 4 5 0 a n d 5 2 0 n m sh o w s a lin e a r d e p e n d e n c e w ith th e m e a n ra d iu s o f th e g o ld p a rtic le s le ss th a n 3 n m ; how ever, th e m e th o d c a n b e u se d to siz e p a rtic le s u p to 7 n m . E x p e rim e n ta l re s u lts for ex tin c tio n sp e c tra c a n b e fitte d b y M ie s th e o ry i f th e o p tic a l co n sta n ts fro m b u lk m a te ria l v alu es a re m o d ifie d b y in tro d u c in g th e lim ita tio n o f th e m e a n fre e p a th d u e to c o llisio n s o f c o n d u c tio n e le c tro n s w ith th e b o u n d a ry o f th e n a n o p a rtic le s.

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This paper develops a parametric method for determining the core radius and shell thickness in small silver–silver-oxide core–shell nanoparticles (Nps) based on single particle optical extinction spectroscopy. The method is based on the study of the relationship between plasmon peak wavelength, full width at half maximum (FWHM) and contrast of the extinction spectra as a function of core radius and shell thickness. This study reveals that plasmon peak wavelength is strongly dependent on shell thickness, whereas FWHM and contrast depend on both variables. These characteristics may be used for establishing an easy and fast stepwise procedure to size core–shell NPs from single particle absorption spectrum. The importance of the method lies in the possibility of monitoring the growth of the silver-oxide layer around small spherical silver Nps in real time. Using the electrostatic approximation of Mie theory, core–shell single particle extinction spectra were calculated for a silver particle's core size smaller than about 20 nm and different thicknesses of silver oxide around it. Analysis of the obtained curves shows a very particular characteristic of the plasmon peak of small silver–silver-oxide Nps, expressed in the fact that its position is strongly dependent on oxide thickness and weakly dependent on the core radius. Even a very thin oxide layer shifts the plasmon peak noticeably, enabling plasmon tuning with appropriate shell thickness. This characteristic, together with the behaviour of FWHM and contrast of the extinction spectra can be combined into a parametric method for sizing both core and shell of single silver Nps in a medium using only optical information. In turn, shell thickness can be related to oxygen content in the Np's surrounding media. The method proposed is applied to size silver Nps from single particle extinction spectrum. The results are compared with full optical spectrum fitting using the electrostatic approximation in Mie theory. The method may be the basis for developing a plasmonic sensor for O2 concentration based on Ag single NP spectroscopy.