Journal of Photochemistry and Photobiology C: Photochemistry Reviews
Invited reviewStudies on the interaction of pulsed lasers with plasmonic gold nanoparticles toward light manipulation, heat management, and nanofabrication
Highlights
► Mechanistic aspects of pulsed-laser-induced size reduction of colloidal gold nanopaticles. ► Interaction of pulsed lasers with gold nanopaticles leading to material fabrication and processing. ► Laser-induced photothermal process and related phenomena of gold nanoparticles. ► Plasmonic field enhancement of gold nanoparicles leading to various photochemical applications.
Introduction
Past decades have witnessed an ever increasing interest in the optical properties of nanoparticles and nanostructures [1], [2], [3], [4], [5], [6], [7], which has had a tremendous impact on broad areas of research such as optics, electronics, biomedicine, analytical chemistry, and photochemistry. Scheme 1 gives an overview of the areas of current promising applications of plasmonic nanoparticles (NPs).
In nano-optics and nanoelectronics, miniaturization with high throughput is highly desirable; consequently, an interest in surface plasmons in optical circuits below the diffraction limit has grown rapidly [8], [9]. In biomedical applications, because of their strongly resonant light-absorbing and light-scattering properties, plasmonic NPs can be useful contrast agents in the diagnostic imaging of tumors [10], [11]. In addition, when illuminated, plasmonic NPs can serve as nanoscale heat sources, photothermally inducing cell death and tumor remission [12], [13]. In chemistry, in particular for analytical applications, surface-enhanced Raman scattering spectroscopy (SERS) utilizing enhanced local electromagnetic fields near the plasmonic NPs and nanostructures is a promising tool for ultratrace analysis, ultimately enabling detection at the single-molecule level [14], [15]. Biosensing that exploits the detection of a refractive-index change around the plasmonic NPs is another area of development [16], [17], [18]. Recent developments have greatly improved the sensitivity of plasmon sensors based on the single NPs and NP arrays. In the plasmonic NP field, photochemistry is currently prominent among the areas receiving increasing attention.
Photovoltaics, which is a method of converting sunlight into electricity, is a major field in photochemistry [19], [20], [21]. Approaches based on plasmonic structures can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the thickness of absorber layers and yielding new options for solar cell design [22], [23]. An effect of plasmon-assisted optical antennas [24], [25], where light is concentrated into a subwavelength volume to convert the incident light into amplified localized fields, has the potential to promote low-yield photochemical reactions with greater efficiency. It has been shown that two-photon-induced photochromic and photopolymerization reactions can be realized under the illumination of continuous wave (CW) lasers and halogen lamps [26], [27], [28], [29]. Previously, these two-photon reactions have only been conducted by irradiation with high-intensity pulsed lasers. Fundamental aspects and applications of metal-enhanced fluorescence [30], [31] and plasmon-assisted photocatalysts [32], [33] are also active areas of research in photochemistry.
This review deals with phenomena initiated by the interaction of lasers with plasmonic NPs that give rise to rich physics and chemistry, as summarized in Scheme 2.
Briefly, the absorption of visible light by a plasmonic NP results in the instantaneous heating of the particle [34], [35]. This heating has a profound effect both on the particle itself and the surrounding medium. The former can be described by electron dynamics [4], [5], [6], [7], coherent acoustic lattice vibrations [6], [36], [37], melting and evaporation from the particle surface [38], [39], and explosive fragmentation [40], [41]. The latter is described by cavitation [42], [43], [44], [45] and stress-wave generation [46], [47], resulting from heat transfer from the particle to the surroundings. These phenomena are intriguing from a fundamental scientific viewpoint and cover a wide range of applications as will be described below (Section 4). For instance, local heating triggered by the light absorption of a plasmonic NP enables nanofabrication on particle support. The amount of heat generated by the plasmonic NP can be controlled by the particle size, shape, and aggregation state of the NP as well as the illuminating laser intensity, wavelength, and pulse duration. Thus interplay of plasmonic NPs with laser light allows spatial and temporal heat management besides the optimization of local electromagnetic field.
This review is intended to provide photochemists with information that is useful when initiating research using plasmonic NPs, in particular with lasers combined with traditional organic and inorganic photochemistry. The review focuses on metal NPs rather than semiconductor quantum dots [48], [49], which are more familiar to photochemists. The photochemistry of noble metal nanoparticles is different from that of traditional molecules. Generally, the fluorescence emission of Au NPs is very weak unless the particle diameters are less than 5 nm [50]. On the other hand, Au NPs with diameters greater than 20 nm scatter light in the dark field and can be used for imaging just like fluorescent dyes or quantum dots without suffering from photobleaching/blinking [51], [52]. Their particle size-, shape-, spacing- and medium-dependent optical properties have distinct advantages that cannot be replaced by any other materials. Note, however, that the accepted method of describing the optical properties of these metal NPs is quite different from that by which the photophysics of other molecules is delineated. This is because, for metals, frequency (wavelength)-dependent responses of dielectric functions are important. In this review, we examine the emerging field of laser–plasmonic NP interactions, leading to light manipulation, heat management, and nanofabrication. Particular emphasis is placed on Au NPs interacting with pulsed lasers. Although Ag NPs have superior characteristics in terms of plasmonic enhancement [53], Au NPs have been selected because they have higher chemical stability and are used more frequently [54], [55].
Section snippets
LSPR: localized surface plasmon resonance [1–6]
Plasmonic NPs absorb and scatter light from UV to near IR. The classical interaction of NPs with light and the resultant optical properties are described by the dielectric function ɛ; here ɛ is usually represented as a function of the angular frequency ω (=2πc/λ) of light. The dielectric function of gold is defined as the sum of the interband term, ɛIB(ω), which considers the response of 5d electrons to the 6sp conduction band, and the Drude term, ɛD(ω), which considers free conduction
Shape transformation
Pulsed-laser irradiation of Au nanorods suspended in water resulted in a shape change to spherical particles [38], [39], [80], [81], [82]. The shape transition of the nanorods was studied by Chang et al. [80] by exposure to 532- and 1064-nm nanosecond (6 ns) lasers. They ascribed the observed rod-to-sphere conversions to a photoannealing process. Link and co-workers carried out a similar experiment by applying both femtosecond (800 nm, 100 fs) and nanosecond (355 nm, 7 ns) lasers [38], [39], [81].
Surface modification and treatment of substrate
Interaction of lasers with plamonic NPs allows nanofabrication on various substrates on which the particles are placed or assembled. Ko and co-workers reported the fabrication of nanometer-sized craters on a polyimide film self-assembled with Au NPs when exposed to a 532-nm nanosecond-laser light through an objective lens (3–5 ns pulse width, 30–300 mJ cm−2 power density) [156]. Tsuboi and co-workers observed the nanohole (d < 100 nm) formation on a Au NP–polymer hybrid film deposited on glass
Summary and future outlook
This review demonstrates the immense potential of Au NPs when used with lasers, providing multiple varieties of research opportunities, many of which are ongoing. The beneficial outcome of the researches can influence various fields of science and technology: physics, chemistry, biomedicine, and materials science. The reader may find the key word “LSPR” repeated many times in this review. LSPR is primarily characterized not only by its distinct color but also by its highly efficient
Acknowledgements
KAKENHI (no. 23310065 and no. 22655043) to S. H. from the Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged. JSPS fellowship to D. W. is gratefully acknowledged. Professor Hiroshi Masuhara is acknowledged for his continuous support and encouragement.
Shuichi Hashimoto has been a Professor of Ecosystem Engineering, at the Graduate School of Technology and Science, The University of Tokushima, Japan since 2005. He received a PhD in Physical Chemistry from Tokyo Metropolitan University in 1982 and carried out postdoctoral research at the University of Notre Dame under J.K. Thomas from 1982 to 1985. He worked with H. Masuhara as a visiting scholar at Osaka University, investigating the transient-absorption spectroscopy of zeolites in 1993. His
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Shuichi Hashimoto has been a Professor of Ecosystem Engineering, at the Graduate School of Technology and Science, The University of Tokushima, Japan since 2005. He received a PhD in Physical Chemistry from Tokyo Metropolitan University in 1982 and carried out postdoctoral research at the University of Notre Dame under J.K. Thomas from 1982 to 1985. He worked with H. Masuhara as a visiting scholar at Osaka University, investigating the transient-absorption spectroscopy of zeolites in 1993. His current research interests are laser-induced size and shape transformations of plasmonic nanoparticles, microscopy–spectroscopy study of nanoparticles and nanostructures, femtosecond-laser processing of transparent solids, and optical microscopy study of zeolite photochemistry.
Daniel Werner, native of Dresden, Germany, is currently a JSPS postdoctoral fellow at The University of Tokushima. He received PhD in Physical Engineering from The University of Tokushima working with Prof. S. Hashimoto. His current research interest focuses on the nonlinear optical properties of multilayered core–shell nanoparticles, electron dynamics and thermodynamics of laser-heated nanoparticles in liquids, fragmentation mechanisms of noble metal nanoparticles and nanoparticle generation in liquids through a pulsed-laser-ablation technique.
Takayuki Uwada received a PhD in Applied Physics from Osaka University under the guidance of Prof. T. Asahi and Prof. H. Masuhara in 2007. After a year of postdoctoral research at the Hamano Life Science Research Foundation in Kobe, he has been appointed as an assistant research fellow in the Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Taiwan under the guidance of Prof. Masuhara. His current research interests include static and dynamic light-scattering-microspectroscopy investigations of nanomaterials, laser-induced molecular/nanoparticle assembly and crystallization, and metallic nanoparticle plasmon application to biophysics.