Attomole-Level Multiplexed Detection of Neurochemicals in Picoliter Droplets by On-Chip Nanoelectrospray Ionization Coupled to Mass Spectrometry

Abstract

While droplet microfluidics is becoming an effective tool for biomedical research, sensitive detection of droplet content is still challenging, especially for multiplexed analytes compartmentalized within ultrasmall droplets down to picoliter volumes. To enable such measurements, we demonstrate a silicon-based integrated microfluidic platform for multiplexed analysis of neurochemicals in picoliter droplets via nanoelectrospray ionization (nESI)-mass spectrometry (MS). An integrated silicon microfluidic chip comprising downscaled 7 μm-radius channels, a compact T-junction for droplet generation, and an integrated nESI emitter tip is used for segmentation of analytes into picoliter compartments and their efficient delivery for subsequent MS detection. The developed system demonstrates effective detection of multiple neurochemicals encapsulated within oil-isolated plugs down to low picoliter volumes. Quantitative measurements for each neurochemical demonstrate limits of detection at the attomole level. Such results are promising for applications involving label-free and small-volume detection for monitoring a range of brain chemicals.

Highly Localized Chemical Sampling at Subsecond Temporal Resolution Enabled with a Silicon Nanodialysis Platform at Nanoliter per Minute Flows

“…However, previously demonstrated on-chip integration of specially designed nanoelectrospray nozzles using our scaled silicon ND platform 29,30 may enable efficient MS hyphenation of pL-segmented dialysate flows with LOD of just a few attomoles. 31 The proposed and demonstrated scaling strategy (Figure 1b−d) can potentially be extended even further in miniaturizing the sampling area, while keeping a high recovery rate and temporal resolution. With mature silicon microfabrication technology, it is relatively straightforward to reduce the channel cross-section, and hence control the pressure-driven flow rates below 1 nL/min (Figure 1d).…”

Highly localized chemical sampling at sub-second temporal resolution enabled with a silicon nanodialysis platform at exceedingly slow flows

“…Integration with previously demonstrated hyphenation of segmented dialysate flows to sensitive MS methods using nano-ESI 29 or MALDI-MS 30 will enable quantification of small molecules at the level of just a few attomoles 31 . The proposed and demonstrated scaling strategy (Fig.…”

“…Further integration of on-chip droplet segmentation and a delay line for droplet storage enables sampling and manipulation of analytes at volumes of just a few pL. Integration with previously demonstrated hyphenation of segmented dialysate flows to sensitive MS methods using nano-ESI 29 or MALDI-MS 30 will enable quantification of small molecules at the level of just a few attomoles 31 . The proposed and demonstrated scaling strategy (Fig.1B,C,D) can potentially be extended even further in miniaturizing the sampling area, while keeping high recovery rate and temporal resolution.…”

“…The drastic reduction of channel cross-section down to 50µm 2 enabled by our silicon platform and its operation at exceedingly slow nL/min flow rates allows for stable on-chip generation of droplets with volumes as small as just a few pL 24 . On-chip delivery of such droplets to online mass spectrometry (MS) analysis via an integrated nano-electrospray ionization (nano-ESI) nozzle 29 or printing them one-by-one for matrix assisted laser desorption ionization (MALDI) MS 30 , we have previously demonstrated limits of detection (LOD) at the level of just a few attomoles 31 . However, such drastic reduction of flow rates and analyte volumes imposes a specific challenge of interfacing the segmented flows at the ND chip edge.…”

Integrated silicon microfluidic chip for picoliter-scale analyte segmentation and microscale printing for mass spectrometry imaging

“…As fabricated, the microfluidic channels are hydrophilic, hence oil is the segmented phase, while the aqueous phase is continuous. Previous MS measurements 28 of the potential carry-over of analytes between consecutive aqueous plugs have shown that carry-over of analytes in the oil-segmented aqueous flow between aqueous plugs of similar volume and frequency is suppressed by at least 3 orders of magnitude.…”