nanoscale materials and structures for high value applications is an emerging area of nanoscience and nanotechnology . nanomaterials , usually ranging from 1 to 100 nanometers ( nm ) , may provide solutions to technological and environmental challenges in the areas of solar energy conversion , catalysis , medicine and water treatment . among such materials , silver nanoparticles have been intensively studied because of their intriguing optical , electronic , mechanical and bactericidal properties . several techniques have been employed for the synthesis of noble metal nanoparticles , such as gas evaporation , arc plasma , sputtering , electrochemical methods , laser ablation , etc . this communication describes the synthesis of silver nanoparticles by chemical reduction of silver ions by nabh4 . we report herein the formation of ring - shaped silver nanoparticles synthesized in a one - pot experiment . extraordinary optical properties from noble metal such as gold and silver are termed surface plasmon resonance induced by the collective oscillation of electron density . it is known that the optical response of silver nanospheres exhibits a single absorption peak corresponding to surface plasmon resonance at about 400 nm . however , aggregated silver nanospheres give rise to two surface plasmon bands corresponding to transverse and longitudinal resonance . several techniques are already known for the elaboration of silver nanoparticles of different shapes like rods , triangular or hexagonal plates by varying the conditions of reduction and capping agent [ 7 - 10 ] . in contrast , little work has been done on the influence of ph conditions on the production of silver nanoparticles . trisodium citrate was obtained from alfa aesar ; sodium hydroxide , silver nitrate and sodium borohydride were purchased from vwr and used as received . photomicrographs of nanoparticles were obtained with a jeol 7400 fegsem and a jeol 2010 tem operated at 400 kv accelerating voltage . uv  vis spectra of nanoparticle suspensions were recorded with a perkin elmer  25 uv  vis spectrophotometer . seven pill boxes containing 4 ml of 10 m trisodium citrate and variable amounts of sodium hydroxide were maintained at 21  0.5 c under stirring . identical volumes of 5.10 m silver nitrate ( 0.35 ml ) and 10 m nabh4 ( 0.15 ml ) were quickly added simultaneously to each pill box under vigorous stirring . trisodium citrate was obtained from alfa aesar ; sodium hydroxide , silver nitrate and sodium borohydride were purchased from vwr and used as received . photomicrographs of nanoparticles were obtained with a jeol 7400 fegsem and a jeol 2010 tem operated at 400 kv accelerating voltage . uv  vis spectra of nanoparticle suspensions were recorded with a perkin elmer  25 uv  vis spectrophotometer . seven pill boxes containing 4 ml of 10 m trisodium citrate and variable amounts of sodium hydroxide were maintained at 21  0.5 c under stirring . identical volumes of 5.10 m silver nitrate ( 0.35 ml ) and 10 m nabh4 ( 0.15 ml ) were quickly added simultaneously to each pill box under vigorous stirring . numerous studies describe to the reduction of silver nitrate in the presence of ammonium hydroxide   in which nh3 plays the double role of base and silver ion complexing agent . in contrast , very little interest has been devoted the influence of sodium hydroxide on the reduction of silver nitrate to obtain silver nanoparticles . so , we decided to investigate systematically the influence of ph on the reduction of silver nitrate by nabh4 ( table 1 ) . reactions were realized at 21  0.5 c by quick and simultaneous addition of silver nitrate and nabh4 under vigorous stirring to the solutions of trisodium citrate containing various amounts of naoh . tested ph ranged from 7.7 to 9.8 . ph values of the different mixtures each assay contained 4 ml of sodium citrate ( 10 m ) , 0.35 ml of agno3 ( 5.10 m ) , 0.15 ml of nabh4 ( 10 m ) and , from 1 to 7 , increasing volumes of freshly made 1 m naoh colors of nanoparticle suspensions changed in the function of reaction ph : scattered light ( fig . 1a ) varied from yellow to green with increasing ph , while transmitted light changed from yellow to purple through dark blue ( fig . sample 1 ( ph 7.7 ) presents a plasmon resonance peak at 400 nm as reported in the literature . samples 2 , 3 and 4 corresponding to ph 8.49.3 ( table 1 ) present a plasmon resonance peak at the same wavelength but with a much more intense absorbance . then , increasing ph to 9.69.75 leads to a decrease in 400 nm absorbance , to the level of the first sample , and to the appearance of an additional broad band around 660 nm . a colors observed of scattered light ; b colors of transmitted light ; c uv  vis spectra ( for conditions see table 1 ) transmission electron microscopy ( tem ) was used to visualize the size and shape of the resulting silver nanoparticles ( fig . reaction mixtures from ph 8.49.7 showed a great abundance of nanorings ( fig . 2c ) . 2b histogram , these nanorings have a mean external diameter of 70 nm and an internal diameter of 30 nm . the aggregation of the nanorings shown in fig . xrd spectra showed a diffraction pattern characteristic of the face - centered - cubic structure of crystalline metallic silver . a , c tem photomicrograph of silver nanoring suspension 2 ; b histogram of nanoring external diameters ; d xrd of nanorings ; e tem photomicrograph of silver nanoring suspension 7 experimental conditions strongly influenced nanoparticle characteristics . for example , in these series of experiments , nanorings were obtained at temperatures comprised between 19 and 22 c . at 25  0.5 c and the same concentration conditions , spherical 3 ) . fegsem photomicrograph of silver spherical hollows suspension obtained at 25  0.5 c to elucidate the mechanism of such nanoring , cryo fegsem experiments were conducted . a solution of citrate 4 ml ( 10 m ) and naoh 70 l ( ph 8.5 ) was stirred vigorously , and 0.35 ml of agno35.10 m was quickly added . before the reduction stage , a drop of the solution was quickly cryogenized by quenching into liquid nitrogen . scanning electron microscopy shows 4 shows a nanoring shape that could be due to an aggregation of small ag2o nanoparticles resulting from the reaction of naoh on silver nitrate . we have realized for the first time a one - pot synthesis and structure characterization of silver nanorings . uniformly sized silver nanorings are characterized by well - defined crystalline structure along the whole ring as shown by xrd patterns . these crystalline structures have unique plasmonic properties that would find applications in nanoscaled photonics , plasmonic devices and optical manipulation . we acknowledge dr . m. guilloton for help in editing this manuscript and the  conseil rgional du limousin for financial support . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited . this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
silver colloidal nanorings have been synthesized by reducing silver ions with nabh4 in trisodium citrate buffers . 
 ph increase , by addition of naoh , was used to speed up reduction reaction . 
 the uv  vis absorption spectra of resulting silver nanorings showed two peaks accounting for transverse and longitudinal surface plasmon resonance , at 400 nm , and between 600 and 700 nm , respectively . 
 the shapes of these silver nanoparticles ( nanorings ) depended on agno3/nabh4 ratio , ph and reaction temperature . 
 particles were analysed by transmission electron microscopy , scanning electron microscopy and x - ray diffraction . 
 a reaction pathway is proposed to explain silver nanoring formation .