(a) Centrifugal biodisk and (b) BioCD systems. Centrifugal biodisks are microfluidic lab-on-a-chip (or lab-on-a-CD) systems that use noninertial forces for fluid pumping and switching to manipulate and distribute fluids. The BioCD systems are used for optical detection of bound molecular films captured by recognition molecules such as antibodies spotted onto the disk surface.
A hydrophobic barrier uses a small hydrophobic capillary to keep liquid in the larger channel until sufficient centrifugal pressure is applied to overcome the capillary pressure of the small restriction. A hydrophilic barrier is a metastable configuration in which all the channel walls have the same contact angle. The capillary pressure keeps the liquid in the small tube until sufficient centrifugal pressure is applied to reverse the curvature to allow expansion into the large channel. Redrawn from Ref. 27.
A Coriolis valve. The force on the flow changes sign when the spin direction reverses. This allows fluid to drain in one direction for one spin direction and the other for the opposite spin direction. Redrawn from Ref. 44.
Dipole scattering near a surface using image scattering in which the interface is replaced by two dipoles and two counterpropagating plane waves.
The phase-quadrature condition between a signal and a reference field. When the relative phase between the signal and reference is , then a phase modulation on the signal is in-phase with the reference wave and is transduced into an intensity modulation on the combined field.
(a) Calculated IL response and (b) DPC response to 1 nm protein layer as a function of the modulus and the phase of . In the calculation, it is assumed that the incident angle is 30° (-polarized) at a wavelength of 488 nm. Redrawn from Ref. 24.
Patterned avidin on a dielectric mirror with an antinode condition detected using DPC laser scanning. The data are unfiltered raw tracks stacked into a 2D representation that gives the impression of 3D. Replotted from Ref. 125.
Side-band demodulation of DPC data of patterned avidin on a dielectric reflecting mirror. The raw data are shown in (a), and the demodulated data are shown in (b). The demodulation removes the periodic stripe pattern and replaces it with the average stripe height with a scaling mass density of 1.5 pg/mm.
Thermal oxide on silicon. When the oxide thickness is an eighth-wave, there is a phase difference between the top and bottom reflections (reflection coefficient is purely imaginary). This establishes a quadrature condition that converts the phase load of a thin protein layer directly into intensity at the far-field detector.
(a) Reflectance as a function of wavelength for three different oxide thicknesses on silicon. (b) The relative change in reflectance as a function of wavelength for the three oxide thicknesses in response to 1 nm of bound protein. The reflectance change is approximately 2% per nanometer of bound protein.
High-resolution interferometric scans of two different antibody spots printed on butyraldehyde-functionalized silica surfaces. Bar is . The top is a chicken IgY and the bottom is a goat IgG. The average spot height is approximately .
IL and DPC scans of graphene sheets adsorbed on thermal oxide on silicon at a wavelength of 532 nm. The refractive index of the graphene film (there is a monolayer and a trilayer in these data) is obtained by combining the data from both phase-contrast and IL channels. Reprinted from Ref. 122.
Response of a ridge-based interferometer to immobilized protein. The ridge in (a) performs as a wavefront splitting interferometer, with half intensity on the ridge and half on the land. The intensity along the optic axis exhibits an ideal two-wave response, shown in (b) as a function of ridge height. The far-field diffraction is shown in (c) for different spoke heights, and the change in intensity upon protein immobilization is shown in (d).
Reflectance as a function of time for a microdiffraction BioCD for bare gold ridges and after antibody immobilization and antigen capture. Comparison of gold ridge heights of in (a) and in (b), illustrating opposite quadratures, decreasing or increasing intensity upon protein binding, respectively. From Ref. 23.
Differential immobilization of protein on gold spokes. (a) The disk image shows FITC-conjugated antibody immobilized on alternating spokes. (b) The intensity as a function of time as the disk spins shows a clear half-harmonic. (c) The power spectrum has a clear peak caused by the alternating protein signal with a signal-to-noise of approximately 300:1.
Two-analyte experiment to detect mouse IgG antimouse IgG binding and rabbit IgG antirabbit IgG binding. Frame 1: printed mouse IgG. Frame 2: after global incubation with rabbit IgG. Frame 3: bands B and C were exposed to antirabbit IgG. Frame 4: bands C and D were exposed to antimouse IgG. Bands A and E were reference bands. The schematic of the experiment is shown on the right. Redrawn from Ref. 127.
Experimental layout using the 488 nm line from an argon laser incident at 30° and focused on the BioCD. The interferometric signal is detected in the reflected light, while the fluorescence signal is collected by a lens above the disk. The oblique-incidence design spatially separates the two channels.
Power spectra of simultaneous fluorescence and interferometry of printed protein stripes on an IL quadrature BioCD. The square-wave protein pattern produces many harmonics. The fluorescence has noticeably lower background than interferometry.
Two-channel scans performed continuously on the same track consisting of antibody conjugated with fluorescein after a reverse-phase assay. The interferometry wavelength is 488 nm, and the fluorescence wavelength is 510 nm. (a) shows the time-course scanning results on both channels as a function of position and time. The fluorescence becomes weaker with time (increasing downward) due to bleaching. (b) shows the signal intensity variations. From Ref. 123.
Comparison of forward and reverse assays using the two channels for interferometry and fluorescence. The interferometry was performed at 488 nm and the fluorescence was at 510 nm. The reverse assay shows a strong amplification for concentrations above .
The concentration detection limit for rabbit IgG is plotted with the detection area. Both interferometric and fluorescent detections have power-law dependence on the area, respectively, with exponents −0.45 and −0.40 (from Ref. 145).
Unit cell structure of the antibody spots. The target spots are active antibodies seeking target analyte molecules in sample. The reference spots are isotype antibodies that are not specific to the target molecule. A well typically has a array of unit cells. The interferometric scans show the prescan antibody height and the additional height upon binding target molecules after the assay binding.
Equilibrium reverse-phase assay capturing antibody out of solution. Each incubation was for 20 h at increasing concentration. The equilibrium constant is 35 ng/ml with a vertical dynamic range of 300:1 and a 16% active fraction of antigen.
Interferometric postscan of a well incubated with 300 ng/ml of human haptoglobin. The strong target spot response is seen relative to the reference spots.
Standard curve for a haptoglobin assay development kit. The wells have unit cells composed of protein A/G that bind specific antibodies and nonspecific IgY antibodies.
Concentration recovery performed on the standard curve in Fig. 27. The recovered concentration is plotted against the known concentration.
Comparison of two A/G immobilization disks against two direct-printed antibody disks. The antibody activity on the protein A/G is nearly 100% and highly repeatable with a near 120 ng/ml. The direct print shows lower activity around 12% but with a lower near 20 ng/ml.
(a) The histogram of thickness increments of 25 000 spots incubated at 10 ng/ml after secondary antibody incubation. Based on the mean values and standard deviations of the distributions, the detection limit of one pair of antibody and reference spots is estimated to be 1.69 ng/ml in 2 mg/ml background concentration. (b) The scaling PSA detection limits of the sandwich assay are shown. The scaling detection limit is fit by a power law with an exponent −0.44. A 20 pg/ml detection limit for the PSA sandwich assay is achieved based on 11 520 antibody spots or 250 pg/ml based on 45 spots.
Concentration recovery of PSA concentrations for three patient samples. The standard curve in serum is on the left, and the dilution curves for the three patients are on the right. The recovered concentrations are 30, 50, and 5000 ng/ml.
BioCD quadrature classes.
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