Aqueous Microlenses for Localized Collection and Enhanced Raman Spectroscopy of Gaseous Molecules

Abstract

light extraction in organic light-emitting diodes, [4,5] miniaturized imaging systems, [6][7][8] optical manipulation, [9] and spectral amplification. [10,11] Microlenses are generally solid-state objects [1][2][3][4][5][6][7][8]10] with fixed shapes whose interactions with chemical environments have been considered both insignificant and undesirable. In contrast, liquid lenses have tunable shapes that enable focal length control and operation in flexible systems. [12][13][14] Liquid lenses are usually made of non-volatile organic compounds [15][16][17][18] and encapsulated to ensure minimal exposure to the environment. [12][13][14]19] An aqueous microlens in the open air is expected to absorb gaseous molecules that are water-soluble and locally concentrate them within the lens. Therefore, aqueous microlenses can be unique optical tools for the collection and analysis of gaseous molecules in the open air. The application of aqueous lenses to gas analysis, however, has not been demonstrated because of poor long-term lens stability resulting from the evaporation of water [3,20,21] and the lack of methods for fabricating them in a well-controlled manner.Raman spectroscopy enables the label-free and reliable identification of various molecular species based on unique molecular fingerprints. Identifying gas-phase molecules by Raman spectroscopy, however, is challenging compared to identifying them in condensed phases because low-density gases at atmospheric pressure provide weak signals. [22,23] As a result, there exist very few studies that report the Raman spectroscopy of volatile organic compounds (VOCs) in the gas phase, [24][25][26][27][28] especially ones that are lethal at low concentrations, such as chemical warfare agents (CWAs). [29][30][31] Spectroscopy of gaseous molecules based on localized collection has been reported using different approaches, such as electrodynamic precipitation, [32][33][34][35] and the use of graphene plasmon [36] and microfluidic [37,38] devices; however, these approaches require complicated experimental procedures and peripheral devices. Electronic sensors based on nanomaterials [39][40][41] are highly sensitive to VOCs including CWAs, but identifying such gaseous analytes has been challenging, even after surface treatment with receptors. [36,42,43] Therefore, developing a technology that enables the Raman spectroscopy of gaseous molecular species is critical for identifying chemicals in the environment.Raman spectroscopy of gaseous molecules has been challenging, requiring complicated experimental procedures and peripheral devices for concentrating the analytes. Here, Raman spectroscopy of gaseous molecules at parts-per-billion (ppb) levels is demonstrated using aqueous microlenses of LiCl solution that spontaneously absorb water-soluble gas molecules from the environment. The lenses are easily formed by filling the microwells of an elastomeric stamp with an aqueous solution of LiCl and stamping onto a substrate. Because LiCl is hygroscopic, the aqueous lenses maintain their liqu...

Interference micro/nanolenses of salts for local modulation of Raman scattering

“…1d), as similarly reported in the previous studies of aqueous microlenses on SiO 2 substrate. 29,30 The Raman intensity modulated by the LiCl microlens is strongly correlated with the Newton rings. As shown in Fig.…”

“…[3][4][5][6][33][34][35][36] For carbon nanotubes, however, the magnitude of the Raman signal is governed by strong resonance-Raman scattering conditions. Therefore, the interference effect in the Raman scattering of carbon nanotubes has been somewhat overlooked although recent studies 29,30 of hygroscopic salt lenses suggest crucial role of interference.…”

“…These micro/nano droplets, with their high surface-to-volume ratio, can capture gaseous molecules from the environment, enabling Raman analysis of those molecules. 30 Several liquid-state micro/nanodroplets share some of the aforementioned advantages. Examples include the formation of ionic liquid 37 or PMMA 38 droplets along a nanowire by molecular transport, as well as the controlled formation of oil nanodroplets 12,13 for various applications including Raman spectroscopy.…”

Recent Trends in SERS-Based Plasmonic Sensors for Disease Diagnostics, Biomolecules Detection, and Machine Learning Techniques

“…A reproducible (7%), rapid (30 s), and sensitive (1 ppb) was used for the detection of a nerve simulant, pinacolyl methyl phosphonic acid (PMPA) [ 334 ]. Gaseous warfare agents such as dimethyl methylphosphonate were identified using SERS on LiCl microlenses [ 335 ]. Various G-series and VX nerve agents were identified using novel pinhole shell-isolated Au nanoparticles substrates achieving sensitivity of 10 ng/L and 20 ng/L, respectively [ 336 ].…”

A facile method for fabricating microlens array with diverse morphologies at general thermal reflow conditions

“…n the past decades, microlens arrays (MLAs) have played an important role in a wide range of applications, including imaging, 1,2) sensing, 3,4) photovoltaic devices, 5) nano energy, 6) organic light emitting devices (OLED), 7) 3D display, 8) and bioelectronics, 9) etc. For different applications, MLAs with different morphologies such as spherical, aspheric and tilted asymmetric MLAs are required.…”