Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Albert-Einstein-Allee 11, 89075 Ulm, Germany.
Analyst. 2012 May 21;137(10):2322-7. doi: 10.1039/c1an15787f. Epub 2012 Jan 17.
We demonstrate ultra-sensitive chemical sensing in the mid-infrared spectral regime with a combination of quantum cascade lasers (QCLs) with GaAs/Al(0.2)Ga(0.8)As strip waveguides fabricated via metal-organic vapor-phase epitaxy (MOVPE) and reactive ion etching (RIE) using evanescent field absorption spectroscopy. These strip waveguides have been designed with a width of 200 μm, thereby facilitating 2-D confinement and mode-matched propagation of mid-infrared radiation emitted from a distributed feedback (DFB) QCL at a wavelength of 10.3 μm. Acetic anhydride was detected with a limit of detection (LOD) of 18 pL (19.4 ng) deposited at the waveguide surface by overlapping of the vibrational absorption of the methyl group with the emission frequency of the QCL. The obtained results indicate a remarkable enhancement in sensitivity by three orders of magnitude compared to previously reported multimode planar silver halide waveguides. Further reduction of the waveguide strip width to 50 μm resulted in an additional sensitivity enhancement yielding a calculated LOD of 0.05 pL for the exemplary analyte acetic anhydride, which is among the most sensitive evanescent field absorption measurements with a miniaturized mid-infrared sensor system reported to date.
我们通过金属有机气相外延（MOVPE）和反应离子刻蚀（RIE），结合量子级联激光器（QCL）和 GaAs/Al(0.2)Ga(0.8)As 条型波导，在中红外光谱范围内展示了超高灵敏度的化学传感。这些条型波导的设计宽度为 200 μm，从而促进了分布式反馈（DFB）QCL 在 10.3 μm 波长处发射的中红外辐射的 2-D 限制和模式匹配传播。通过将甲基的振动吸收与 QCL 的发射频率重叠，在波导表面沉积的 18 pL（19.4 ng）乙酸酐可检测到其存在，检测限（LOD）为 18 pL（19.4 ng）。与之前报道的多模平面卤化银波导相比，这一结果表明灵敏度显著提高了三个数量级。进一步将波导条的宽度减小到 50 μm，会产生额外的灵敏度增强，对示例分析物乙酸酐的计算 LOD 为 0.05 pL，这是迄今为止报道的使用小型化中红外传感器系统进行的最灵敏的消逝场吸收测量之一。
