Surface-enhanced Raman scattering (SERS) is a powerful spectroscopy technique that can provide non-destructive and ultra-sensitive characterization down to single molecular level, comparable to single-molecule fluorescence spectroscopy. However, generally substrates based on metals such as Ag, Au and Cu, either with roughened surfaces or in the form of nanoparticles, are required to realise a substantial SERS effect, and this has severely limited the breadth of practical applications of SERS. A number of approaches have extended the technique to non-traditional substrates, most notably tip-enhanced Raman spectroscopy (TERS) where the probed substance (molecule or material surface) can be on a generic substrate and where a nanoscale gold tip above the substrate acts as the Raman signal amplifier. The drawback is that the total Raman scattering signal from the tip area is rather weak, thus limiting TERS studies to molecules with large Raman cross-sections. Here, we report an approach, which we name shell-isolated nanoparticle-enhanced Raman spectroscopy, in which the Raman signal amplification is provided by gold nanoparticles with an ultrathin silica or alumina shell. A monolayer of such nanoparticles is spread as 'smart dust' over the surface that is to be probed. The ultrathin coating keeps the nanoparticles from agglomerating, separates them from direct contact with the probed material and allows the nanoparticles to conform to different contours of substrates. High-quality Raman spectra were obtained on various molecules adsorbed at Pt and Au single-crystal surfaces and from Si surfaces with hydrogen monolayers. These measurements and our studies on yeast cells and citrus fruits with pesticide residues illustrate that our method significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for the inspection of food safety, drugs, explosives and environment pollutants.

Flexible nanogenerators that efficiently convert mechanical energy into electrical energy have been extensively studied because of their great potential for driving low-power personal electronics and self-powered sensors. Integration of flexibility and stretchability to nanogenerator has important research significance that enables applications in flexible/stretchable electronics, organic optoelectronics, and wearable electronics. Progress in nanogenerators for mechanical energy harvesting is reviewed, mainly including two key technologies: flexible piezoelectric nanogenerators (PENGs) and flexible triboelectric nanogenerators (TENGs). By means of material classification, various approaches of PENGs based on ZnO nanowires, lead zirconate titanate (PZT), poly(vinylidene fluoride) (PVDF), 2D materials, and composite materials are introduced. For flexible TENG, its structural designs and factors determining its output performance are discussed, as well as its integration, fabrication and applications. The latest representative achievements regarding the hybrid nanogenerator are also summarized. Finally, some perspectives and challenges in this field are discussed.

3718 wileyonlinelibrary.com a sustainable power source for powering these devices is a focus of today's research. Various approaches based on piezoelectric, [1][2][3][4] electromagnetic, [ 5,6 ] electrostatic effects [7][8][9][10] have been demonstrated. Besides, many novel liquid-based energy conversion technologies, including reverse electrowetting [ 11 ] and electric-double-layer modulating [ 12 ] are also proposed. Recently, by combining triboelectric effect [ 13,14 ] and the electrostatic induction phenomenon, triboelectric nanogenerator (TENG) is invented. It features in low cost, diversiform material options, and free of precharging, and shows a signifi cantly high power output and a high energy conversion effi ciency up to 55%. [15][16][17][18] Therefore, it enables self-powered, autonomous electronics and potentially large-scale power generation possible. [ 19,20 ] However, all of the TENGs reported today are normally based on solid materials, so that the effectiveness of contact, especially to the nanometer level, can be largely affected by the roughness of the two surfaces and the match between the two. According to the literature, the present TENG's charge density is about 100 µC m −2 . [ 21,22 ] Meanwhile, the solid-solid friction will result in heat generation and dissipation. Those two largely limit the effi ciency of the TENG.The current mostly used low-cost electrodes are aluminum and copper. As a metal material, liquid metal is widely investigated for its outstanding physical capabilities, [ 23,24 ] such as high conductivity and favorable fl exibility, which initiates promising applications in chip cooling, [ 25 ] printed electronics, [ 26,27 ] and energy science (lithium battery, [ 28 ] thermoelectric cell). [ 29 ] Since triboelectrifi cation is a surface charging effect [ 14,30 ] and the liquid-solid contact will potentially introduce larger contact area, higher contact intimacy, and lower friction coeffi cient, with respect to the solid-solid contact, the liquid metal would be an ideal contact material for TENG's contact electrode.In this work, we develop the liquid-metal-based triboelectric nanogenerator (LM-TENG). Operating at a separating velocity of 0.25 m s −1 , the LM-TENG having a contact area of 15 cm 2 could generate a voltage of 679 V and a current of 9 µA. More importantly, its output charge density reaches 430 µC m −2 , Liquid-Metal Electrode for High-Performance Triboelectric Nanogenerator at an Instantaneous Energy Conversion Effi ciency of 70.6%Wei Tang , Tao Jiang , Feng Ru Fan , Ai Fang Yu , Chi Zhang , Xia Cao , * and Zhong Lin Wang * Harvesting ambient mechanical energy is a key technology for realizing selfpowered electronics, which has tremendous applications in wireless sensing networks, implantable devices, portable electronics, etc. The currently reported triboelectric nanogenerator (TENG) mainly uses solid materials, so that the contact between the two layers cannot be 100% with considering the roughness of the surfaces, which greatly reduces the total charge dens...