Surface-enhanced Raman Scattering (SERS) has represented a great advance in the field of the Raman spectroscopy during the seventies since the inherent weakness of the Raman signal can be substantially increased by six or more orders of magnitude. The application of SERS requires the use of mainly Ag, Au and Cu having a nanostructured morphology. In fact, it is crucial the absence on surface of impurities, since SERS technique is so sensitive that very low adsorbate concentrations can also be detected giving spurious bands in the SERS spectra. Most SERS active substrates are metal nanoparticles prepared by chemical reduction using an excess of reductants, such as hydroxylamine hydrochloride or trisodium citrate. Other reagents can be added to colloidal solution either for aggregating the colloidal particles or stabilising them. However, these colloids display several disadvantages which limit their application. In addition, small metal clusters are of great interest because of their optical, electronic and catalytic properties and are useful in wide fields1,2, including biological labelling and photography. Besides the use of conventional chemical and photochemical techniques, ?-radiolysis appears to be a suitable method to form metal colloids in solution.3 ?-irradiation has important advantages as compared to the chemical reduction method, i.e. it does not require the addition of reducing agents since the reduction is performed by radical species formed after interaction of ionising radiation with the solvent. In fact, in aqueous solutions the formation of two short-lived reducing species, solvated electron (e-aq) and hydrogen atoms (oH), is obtained together with hydroxyl radicals (oOH), a strong oxidizing species. Ag nanoparticles were successfully prepared at room temperature by ?-radiolysis of Ag+ aqueous solution containing t-BuOH or i-PrOH which act as oOH scavengers, without the addition of aggregating or stabilizing substances. The metal colloids were characterised by UV/Vis spectroscopy and Scanning Electron Microscopy. Many experimental conditions were tested (i.e. Ag+ concentration, irradiation dose and oOH scavenger alcohol) in order to obtain the best controlled size of nanoparticles as well as the high stability of colloidal silver with time. The use of relatively low irradiation doses and Ag+ concentrations allowed to obtain very stable suspensions of Ag nanoparticles without adding any colloid stabiliser, a source of further spurious bands in the Raman spectra (Figure 1). The suitability of the gamma-irradiated colloids in SERS spectroscopy was tested by using thiram, a known fungicide. Micro-SERS and SERS spectra of good quality were achieved at very low concentration of adsorbate. The control SERS spectrum obtained from one stable colloid, radiolytically generated, shows weak features, as compared to the background spectrum obtained for the conventional citrate colloid (Figure 2). In conclusion, the present study shows how colloid chemistry and free-radical chemistry may be combined, giving rise to colloidal metals useful in SERS spectroscopy.
Silver nanoparticles generated by gamma-irradiation: application in Surface-Enhanced Raman Spectroscopy
A Torreggiani;Maurizio Tamba;M D'Angelantonio;
2009
Abstract
Surface-enhanced Raman Scattering (SERS) has represented a great advance in the field of the Raman spectroscopy during the seventies since the inherent weakness of the Raman signal can be substantially increased by six or more orders of magnitude. The application of SERS requires the use of mainly Ag, Au and Cu having a nanostructured morphology. In fact, it is crucial the absence on surface of impurities, since SERS technique is so sensitive that very low adsorbate concentrations can also be detected giving spurious bands in the SERS spectra. Most SERS active substrates are metal nanoparticles prepared by chemical reduction using an excess of reductants, such as hydroxylamine hydrochloride or trisodium citrate. Other reagents can be added to colloidal solution either for aggregating the colloidal particles or stabilising them. However, these colloids display several disadvantages which limit their application. In addition, small metal clusters are of great interest because of their optical, electronic and catalytic properties and are useful in wide fields1,2, including biological labelling and photography. Besides the use of conventional chemical and photochemical techniques, ?-radiolysis appears to be a suitable method to form metal colloids in solution.3 ?-irradiation has important advantages as compared to the chemical reduction method, i.e. it does not require the addition of reducing agents since the reduction is performed by radical species formed after interaction of ionising radiation with the solvent. In fact, in aqueous solutions the formation of two short-lived reducing species, solvated electron (e-aq) and hydrogen atoms (oH), is obtained together with hydroxyl radicals (oOH), a strong oxidizing species. Ag nanoparticles were successfully prepared at room temperature by ?-radiolysis of Ag+ aqueous solution containing t-BuOH or i-PrOH which act as oOH scavengers, without the addition of aggregating or stabilizing substances. The metal colloids were characterised by UV/Vis spectroscopy and Scanning Electron Microscopy. Many experimental conditions were tested (i.e. Ag+ concentration, irradiation dose and oOH scavenger alcohol) in order to obtain the best controlled size of nanoparticles as well as the high stability of colloidal silver with time. The use of relatively low irradiation doses and Ag+ concentrations allowed to obtain very stable suspensions of Ag nanoparticles without adding any colloid stabiliser, a source of further spurious bands in the Raman spectra (Figure 1). The suitability of the gamma-irradiated colloids in SERS spectroscopy was tested by using thiram, a known fungicide. Micro-SERS and SERS spectra of good quality were achieved at very low concentration of adsorbate. The control SERS spectrum obtained from one stable colloid, radiolytically generated, shows weak features, as compared to the background spectrum obtained for the conventional citrate colloid (Figure 2). In conclusion, the present study shows how colloid chemistry and free-radical chemistry may be combined, giving rise to colloidal metals useful in SERS spectroscopy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
