Asymmetric nanofluidic grating detector for differential refractive index measurement and biosensing

Abstract

Measuring small changes in refractive index can provide both sensitive and contactless information on molecule concentration or process conditions for a wide range of applications. However, refractive index measurements are easily perturbed by non-specific background signals, such as temperature changes or non-specific binding. Here, we present an optofluidic device for measuring refractive index with direct background subtraction within a single measurement. The device is comprised of two interdigitated arrays of nanofluidic channels designed to form an optical grating. Optical path differences between the two sets of channels can be measured directly via an intensity ratio within the diffraction pattern that forms when the grating is illuminated by a collimated laser beam. Our results show that no calibration or biasing is required if the unit cell of the grating is designed with an appropriate built-in asymmetry. In proof-of-concept experiments we attained a noise level equivalent to ∼10 refractive index units (30 Hz sampling rate, 4 min measurement interval). Furthermore, we show that the accumulation of biomolecules on the surface of the nanochannels can be measured in real-time. Because of its simplicity and robustness, we expect that this inherently differential measurement concept will find many applications in ultra-low volume analytical systems, biosensors, and portable devices.

Demonstration of a Low-Cost and Portable Optical Cavity-Based Sensor through Refractive Index Measurements

“…The LOD of the differential value was 1.73 × 10 −5 RIU which is 45% and 37% better than that of 830 nm and 880 nm, respectively. This LOD is comparable with other methods reported that are neither low-cost nor portable [18,19,20,21,22,23,24].…”

Nanofluidic Immobilization and Growth Detection of Escherichia coli in a Chip for Antibiotic Susceptibility Testing

“…The derivation and calculation of the signal have been described previously and can be summarized in Equation (1) [ 27 ]. The common mode rejection of this measurement principle was shown before [ 26 ]. …”

“…To characterize the fluidic chip, we conducted calibration measurements with different glycerol dilutions in deionized DI water with concentrations of 0 %, 2% and 4% ( w / w ), which represents a refractive index range of n = 1.3300–1.3377. The refractive index depends on the concertation of the solution [ 26 ]. Prior to the measurement, the detection and the reference channels were filled with DI water.…”

“…Therefore, a hydrodynamic bacterial cell-trapping principle which has been recently been reported [ 24 ] is now implemented in the form of a regular nanogap array to allow readout with an interferometric method. Here, we present an optofluidic chip which can immobilize bacteria in an optical asymmetric grating to measure the proliferation of bacteria with high sensitivity by interferometry based on optofluidic concepts that, in the past, had been developed to measure surface bound molecules [ 25 , 26 ]. This study describes the fabrication and characterization of the optofluidic nanosystem and shows the applicability of this approach in future ASTs.…”

Advances in Label-Free Detections for Nanofluidic Analytical Devices

“…In their study, the diffraction efficiency for the diffracted light of the first order was monitored to detect the effective refractive index inside the nanochannels. Purr et al prepared a device with asymmetric interdigitating arrangement of detection and reference nanochannels to directly calculate the refractive index [ 23 ]. This nanofluidic device could also be integrated into a smartphone-based biosensing system [ 24 ].…”