飛秒雷射誘發光學熔損製備金與銀奈米粒子

Abstract

當飛秒雷射脈衝聚焦在水中，會在微米大小的雷射焦點產生一股斥力並向外擴散，這種飛秒雷射誘發光學熔損廣泛地應用在一些很具發展性的科技上，如分離和操作獨立附著的細胞以及嵌入奈米粒子與生物分子進細胞內，近年在飛秒雷射應用的研究上，研究人員只專注在雷射產生的物理作用上，鮮少探討化學方面的作用，例如雷射照射下短暫物質的生成，因此，飛秒雷射在溶液中誘發多光子吸收產生化學反應的研究對於證明雷射誘發的物理作用與化學作用是相當重要的。 在我們的研究中，金奈米及銀奈米粒子的製備可以當成是探測的工具，藉由生成的金屬奈米粒子因表面電漿共振所展現的消光光譜，可證實飛秒雷射照射下所生成的離子和自由基等短暫物質，這些奈米粒子是藉由四氯金酸與硝酸銀在溶液中的多光子還原來製備，奈米粒子膠體溶液的形成可由消光光譜上的變化而得知。 雷射能量相關的作圖可以推測四氯金酸的還原是透過四氯金酸離子的雙光子吸收來進行，對銀奈米粒子的製備而言，消光光譜的強度只在聚乙烯吡咯烷酮存在的溶液中大量增加，從這可以得知聚乙烯吡咯烷酮的雙光子吸收造成還原反應的發生，然而在沒有聚乙烯吡咯烷酮的硝酸銀溶液中仍然可觀測到少量的光譜變化，這結果可推論水的四光子吸收，也就是飛秒雷射誘發光學熔損所生成的氫原子自由基與氫氧自由基可以還原銀離子。 因此，透過四氯金酸、聚乙烯吡咯烷酮及水在雷射照射下的多光子吸收，我們推導出金奈米及銀奈米粒子還原的機制，此外，雷射熔損也可控制奈米粒子的製備，根據不同的實驗參數，我們也成功地控制奈米粒子的大小與粒徑分佈，這研究將會成功且創新地引導未來飛秒雷射誘發現象的發展。

When femtosecond laser pulses are focused into water, an impulsive force is generated that propagates from the laser focal point in a micron-sized space. This femtosecond laser-induced optical breakdown has been widely used as a promising technique for detaching and manipulating individual adherent cells, and insertion of nanoparticles and bio-molecules into cells. In early studies on the femtosecond laser application, most researchers have focused only on the induced physical behaviors, but rarely on chemical aspect of the behaviors such as generation of transient chemical species by the irradiation. The investigation of the chemical reactions on femtosecond laser-induced multiphoton absorption in solution is indispensable to confirm that these physical behaviors are realized without any chemical changes. In this work, gold and silver nanoparticle fabrication is chosen as a probe in order to confirm the generation of transient species such as ions and radicals by intense femtosecond laser irradiation, since the resultant metal nanoparticles shows these characteristic extinction spectra due to their surface plasmon. These nanoparticles are prepared by the multiphoton reduction of HAuCl4 and AgNO3 in the presence/absence of polyvinylpyrrolidone (PVP) and/or 1-propanol, and the formation of colloid formation is confirmed by the change in the extinction spectra. The laser energy dependence of gold nanoparticle formation strongly supports the reduction of HAuCl4 through 2-photon absorption of AuCl4-. On the other hand, for silver nanoparticle formation, the increase in the Δextinction is observed only in the presence of PVP, which indicates that the colloid formation is achieved through the 2-photon absorption of PVP. While, even in the absence of PVP, a little extinction change is observable. This result suggests that silver ions are reduced by hydroxyl and/or hydrogen radicals due to 4-photon absorption of water, that is due to femtosecond laser-induced optical breakdown. Thus, we successfully demonstrate multiphoton absorption of AuCl4-, PVP and H2O by femtosecond laser irradiation as a primary mechanism of gold and silver nanoparticle fabrication. Furthermore, we have succeeded in nanoparticle fabrication by laser ablation of nanoparticles formed by the reduction, and also succeeded in controlling the mean size and the size distribution by optimizing various kinds of experimental conditions. These successes will give us a novel guideline for the studies on femtosecond laser-induced phenomena.