Pushing the Limits of Electrical Detection of Ultralow Flows in Nanofluidic Channels

Abstract

This paper presents improvements in flow detection by electrical cross-correlation spectroscopy. This new technique detects molecular number fluctuations of electrochemically active analyte molecules as they are transported by liquid flow through a nanochannel. The fluctuations are used as a marker of liquid flow as their time of flight in between two consecutive transducers is determined, thereby allowing for the measurement of liquid flow rates in the picoliter-per-minute regime. Here we show an enhanced record-low sensitivity below 1 pL/min by capitalizing on improved electrical instrumentation, an optimized sensor geometry and a smaller channel cross section. We further discuss the impact of sensor geometry on the cross-correlation functions.

Nanoscale Methods for Single-Molecule Electrochemistry

“…At the extremely high surface-to-volume ratios of nanochannels of 10 7 m −1 or larger, however, the role of any residual adsorption is greatly magnified. Measurements in nanogap devices at high concentration indeed find pronounced signatures of adsorption, with the degree of adsorption corresponding to each molecule spending a factor of 2-4 more time adsorbed to the electrodes than freely diffusing (66,(73)(74)(75), consistent with the observed suppression of the single-molecule current in Reference 67. A consequence of this interpretation is that at an electrode spacing of ∼100 nm or less, adsorption becomes the dominant factor slowing down the rate at which a molecule can undergo redox cycling.…”

“…Nanogap sensors consist of two metal electrodes embedded in the floor and ceiling of a nanofluidic channel, as shown in Figure 7b,c. Hundreds of devices can be fabricated simultaneously on a silicon wafer substrate using photolithographic microfabrication (72), and the critical interelectrode distance is defined reliably by the thickness of an evaporated sacrificial Cr layer (66,72).…”

“…Additional possibilities also open when pressure-driven flow is combined with multiple nanogap transducers in a single nanochannel, a system that some of us have recently employed as a sensitive flow meter (66,78). Several subsequent electrode pairs could either improve the reliability of detection if a single molecule is measured repeatedly as it passes along the nanochannel.…”

Thermal-wave balancing flow sensor with low-drift power feedback

“…The decrease of the thermal penetration depth with frequency indicates that faster operation frequencies can be achieved in much smaller devices, measuring e.g. small flows in nanochannels [21].…”

Feedback‐amplified electrochemical dual‐plate boron‐doped diamond microtrench detector for flow injection analysis

“…Generator‐collector electrode systems have attracted attention in electroanalysis , exploiting the advantages of greater sensitivity (due to enhanced mass transport) and greater selectivity (due to additional dual potential control ). In particular novel nanogap generator‐collector devices incorporated in flow systems are ground‐breaking in terms of new analytical information obtained and improved sensitivity . Dual‐plate microtrench electrodes with down to 2 μm interelectrode gap have been introduced recently as a versatile and readily fabricated generator‐collector electrode system with one open side to allow analyte diffusion into the interelectrode space (see Fig.…”