Portable, smartphone-linked, and miniaturized photonic resonator absorption microscope (PRAM Mini) for point-of-care diagnostics.

We report the design, development, and characterization of a miniaturized version of the photonic resonator absorption microscope (PRAM Mini), whose cost, size, and functionality are compatible with point-of-care (POC) diagnostic assay applications. Compared to previously reported versions of the PRAM instrument, the PRAM Mini components are integrated within an optical framework comprised of an acrylic breadboard and plastic alignment fixtures. The instrument incorporates a Raspberry Pi microprocessor and Bluetooth communication circuit board for wireless control and data connection to a linked smartphone. PRAM takes advantage of enhanced optical absorption of ∼80 nm diameter gold nanoparticles (AuNP) whose localized surface plasmon resonance overlaps with the ∼625 nm resonant reflection wavelength of a photonic crystal (PC) surface. When illuminated with wide-field low-intensity collimated light from a ∼617 nm wavelength red LED, each AuNP linked to the PC surface results in locally reduced reflection intensity, which is visualized by observing dark spots in the PC-reflected image with an inexpensive CMOS image sensor. Each AuNP in the image field of view can be easily counted with digital resolution. We report upon the selection of optical/electronic components, image processing algorithm, and contrast achieved for single AuNP detection. The instrument is operated via a wireless connection to a linked mobile device using a custom-developed software application that runs on an Android smartphone. As a representative POC application, we used the PRAM Mini as the detection instrument for an assay that measures the presence of antibodies against SARS-CoV-2 infection in cat serum samples, where each dark spot in the image represents a complex between one immobilized viral antigen, one antibody molecule, and one AuNP tag. With dimensions of 23 × 21 × 10 cm, the PRAM Mini offers a compact detection instrument for POC diagnostics.