Interfacial liquid-state surface-enhanced Raman spectroscopy

Abstract

Oriented assemblies of functional nanoparticles, with the aid of external physical and chemical driving forces, have been prepared on two-dimensional solid substrates. It is challengeable, however, to achieve three-dimensional assembly directly in solution, owing to thermal fluctuations and free diffusion. Here we describe the self-orientation of gold nanorods at an immiscible liquid interface (that is, oleic acid-water) and exploit this novel phenomenon to create a substrate-free interfacial liquid-state surface-enhanced Raman spectroscopy. Dark-field imaging and Raman scattering results reveal that gold nanorods spontaneously adopt a vertical orientation at an oleic acid-water interface in a stable trapping mode, which is in good agreement with simulation results. The spontaneous vertical alignment of gold nanorods at the interface allows one to accomplish significant additional amplification of the Raman signal, which is up to three to four orders of magnitude higher than that from a solution of randomly oriented gold nanorods.

Three-dimensional SERS hot spots for chemical sensing: Towards developing a practical analyzer

“…However, it remains a formidable challenge to robust control over the orientation and position of NPs in 3D geometry because of thermal fluctuations and diffusion [68]. Recently, some novel reports on forming hotspots and traping molecules may provide new perspectives to overcome these difficulties, including, pH-controllable reversible assembly of NPs [69,70], thermo-induced coupling in Au/polymer hybrid plasmonic structures [71], and the capillary force-induced self-conglutination of metallic nanorods [53,54] or nanowires [72].…”

“…Trace levels of analytes could be detected from molecules dissolved in either the aqueous or the oil phases. [68,82] Trace multi-analyte detection has also been demonstrated, including cases when some molecules were dissolvable in aqueous phase and some in the organic phase,…”

Recent progress on colloidal metal nanoparticles as signal enhancers in nanosensing

“…Strategies like magnetic optical accumulation by using hybrid materials [67], nanoparticle assemblies in liquid-liquid or liquid-air interfaces [68,69] or microfluidics and microdroplets techniques [70,71], have effectively demonstrated to overcome such sensitivity issue. SERS ultradetection of a Raman molecular probe (1-naphthalenethiol) was possible by concentrating core-shell γ-Fe 2 O 3 @SiO 2 coated with rod-shape AuNPs in a small spot of the sample containing the multifunctional material by applying a magnetic field [72].…”

Localized fields, global impact: Industrial applications of resonant plasmonic materials

“…Platforms need not be solid-state, and liquid-state substrate-free Raman spectroscopy is an emerging technology. 64 By using the strongly enhanced fi eld between two metals, tip-enhanced Raman spectroscopy (TERS) promises sub-1-nm spectral mapping. 65 At low temperature and high vacuum, the tip-to-surface distance is precisely controlled to match the nanocavity plasmon resonance with the energy of molecular vibrations, which signifi cantly enhances the Raman signal.…”