Non-invasive light scattering sensor for low-cost microbial detection in solutions

Overview

This solution offers a non-invasive, sensitive, and low-cost method for detecting microbial contamination in aqueous solutions such as injectable drugs, infusion fluids, and other medical or pharmaceutical liquids. The approach exploits the fact that clean solutions contain no particles larger than 3 nanometers and therefore do not scatter red or near-infrared light, while microbial particles in the 0.5 to 2 micrometer range produce strong Mie scattering. By designing an inexpensive optical probe that maximizes light–microbial interactions, the device can reveal the presence of contamination quickly and without contacting or consuming the sample.

The technology is designed to complement existing test-strip approaches by providing a complementary optical readout, enabling broader coverage of contamination screening workflows in pharmaceutical manufacturing, hospital compounding, and clinical diagnostics.

Technical specifications

Detection principle:

• Based on Mie scattering theory applied to microbial particles sized 0.5 to 2 micrometers
• Uses red or near-infrared laser wavelengths that pass through clean solutions without scattering
• Microbial cells produce detectable scattered light proportional to their concentration

Optical design:

• Retrofit foam case encloses the entire sample bottle to block ambient light
• Internal facing mirrors form an optical cavity that reflects the laser beam, increasing the interaction volume between light and microbes
• Multiple photodiodes positioned at angles unreachable by the direct or reflected laser capture only microbially scattered light
• Circuitry sums and auto-reads the photodiode signals

Performance characteristics:

• Validated theoretical sensitivity: a scattering coefficient of 3.1 × 10⁻⁵ per millimeter at 100 degrees for 0.7 micrometer microbial particles at a concentration of 10 microbes per milliliter
• Incident laser power of 4 milliwatts (Class 1, eye-safe) over a 50 millimeter optical path produces approximately 0.006 milliwatts of scattered light
• Signal levels are readily detectable by inexpensive germanium or silicon photodiodes, with further gains possible using avalanche photodiodes
• Estimated cost per test under $0.10 using commodity laser diodes and photodiodes

Operation:

• User fits the foam case around a standard bottle, presses a button, and receives a reading
• No sample preparation, reagents, or consumables required
• Fully non-invasive; the solution remains untouched

Technology readiness level

The technology is currently at a theoretical validation stage. Mie scattering calculations have been performed using representative inputs including microbial diameter, refractive indices of the solution and microbial cells, laser wavelength, and concentration, demonstrating that detectable signal levels can be achieved with inexpensive components. The next planned step is to build a physical prototype consisting of a black foam case with embedded laser diode, mirrors, and photodiodes, and to experimentally optimize variables such as wavelength, intensity, phase, and collimation. Further validation will be pursued through coordinated studies with complementary test-strip based detection methods.