摘要/Abstract
金属纳米结构由于其独特的局域表面等离激元共振现象而倍受关注，对催化、传感、纳米医学以及光学器件等具有重要意义.电荷转移等离激元共振强烈依赖于纳米单元间的导电结点，可产生频率连续可调的共振光吸收和光散射，为获得高度局域化的增强光磁场和光热效应提供了可能.然而，受制于已有构筑手段和有限的结构种类，相关研究仍处于初级阶段.针对此，本工作发展了一种十分简单、有效的Au/Cu纳米异质结点调控策略，利用廉价易得的天然DNA分子在金纳米粒子“种子”表面发生非特异性吸附，有效控制铜在金表面发生异相成核时的相间接触面积，得到导电结点宽度连续可调的电荷转移纳米粒子二聚体.实验光谱和理论模拟显示，结点宽度、铜和金纳米粒子的尺寸是决定电荷转移等离激元性质的重要参数，其分别可由DNA吸附量、Cu2+加入量和金纳米粒子尺寸加以控制，进而实现共振波长在可见至近红外区的宽广调节.通过与其它吸附分子对比证明了DNA吸附调控模式的独特性.这种具有可调控导电结点的双金属纳米异质界面为实现电荷转移等离激元共振与催化和传感等功能的集成以及相关应用探索奠定了重要基础.
关键词: 电荷转移, 界面, 金属, 二聚体, DNA
Metal nanostructures with localized surface plasmon resonance (LSPR) have attracted great attention in catalysis, sensing, nanooptics, and nanomedicine. Charge transfer plasmon (CTP) is a LSPR mode that strongly depends on a conductive junction between metallic nanounits. Benefitting from the charge transfer junction, CTP provides a facile way to generate widely tunable LSPR with highly localized/enhanced light magnetic field and photothermal effect. The limited availability of highly tunable CTP structures and their fabrication techniques hinders a further pursuit of their functions and applications. In response to this situation, the present work aims at developing a simple while highly efficient synthetic route to width-adjustable Au/Cu heterojunctions capable of evoking tunable CTP behaviors. The strategy relies on a non-specific surface adsorption of low-cost, naturally occurred fish sperm DNA on a gold nanoseed to control heterogeneous copper nucleation. Such a process offers a chance to tailor the contact area between the gold and copper nano-domains in the bimetallic structure. Highly tunable CTP resonance from visible to near-infrared region is then realizable on the basis of this method. Experimental and calculated extinction spectra consistently reveal three key variables for the CTP structure, including the width of conductive junction and the sizes of gold and copper particles. These parameters are associated with DNA coverage, copper precursor concentration, and the synthetic conditions for gold nanoparticles, which allow for a CTP tuning from visible to near infrared wavelengths. By fully exploiting these highly controllable parameters, the maximally achievable CTP wavelength readily enters a near infrared Ⅱ domain. The resulting CTP signals have a red-shift of up to 750 nm relative to the 530~570 nm LSPR peaks of individual gold and copper nanoparticles, corresponding to a very narrow Au/Cu conductive contact of 11~13 nm in width. The role of nonspecific DNA adsorption in the above process proves unique (currently irreplaceable) compared to other molecular adsorbates. The easily tunable Au/Cu heterointerface paves a way to integrated CTP and catalytic/sensing functions in future research.
Key words: charge transfer, interface, metal, dimerization, DNA
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