Abstract
In single cell analysis (SCA), individual cell-specific properties and inhomogeneous cellular responses are being investigated that is not subjected to ensemble-averaging or heterogeneous cell population effects. For proteomic single cell analysis, ultra-sensitive and reproducible separation and detection techniques are essential. Microfluidic devices combined with UV laser induced fluorescence (UV-LIF) detection have been proposed to fulfill these requirements. Here, we report on a novel microfluidic chip fabrication procedure that combines straightforward production of polydimethylsiloxane (PDMS) chips with a reduced UV fluorescence background (83%-reduction) by using PDMS droplets with carbon black pigments (CBP) as additives. The CBP-droplet is placed at the point of detection, whereas the rest of the chip remains transparent, ensuring full optical control of the chip. We systematically studied the relation of the UV background fluorescence at CBP to PDMS ratios (varying from 1:10 to 1:1000) for different UV laser powers. Using a CBP/PDMS ratio of 1:20, detection of a 100 nM tryptophan solution (S/N = 3.5) was possible, providing a theoretical limit of detection of 86 nM (with S/N = 3). Via simultaneous two color UV/VIS-LIF detection, we were able to demonstrate the electrophoretic separation of an analyte mixture of 500 nM tryptophan (UV) and 5 nM fluorescein (VIS) within 30 s. As an application, two color LIF detection was also used for the electrophoretic separation of the protein content from a GFP-labeled single Spodoptera frugiperda (Sf9) insect cell. Thereby just one single peak could be measured in the visible spectral range that could be correlated with one single peak among others in the ultraviolet spectra. This indicates an identification of the labeled protein γ-PKC and envisions a further feasible identification of more than one single protein in the future.