Quantum dot enabled detection of Escherichia coli using a cell-phone

Abstract

We report a cell-phone based Escherichia coli (E. coli) detection platform for screening of liquid samples. In this compact and cost-effective design attached to a cell-phone, we utilize anti-E. coli O157:H7 antibody functionalized glass capillaries as solid substrates to perform a quantum dot based sandwich assay for specific detection of E. coli O157:H7 in liquid samples. Using battery-powered inexpensive light-emitting-diodes (LEDs) we excite/pump these labelled E. coli particles captured on the capillary surface, where the emission from the quantum dots is then imaged using the cell-phone camera unit through an additional lens that is inserted between the capillary and the cell-phone. By quantifying the fluorescent light emission from each capillary tube, the concentration of E. coli in the sample is determined. We experimentally confirmed the detection limit of this cell-phone based fluorescent imaging and sensing platform as ∼5 to 10 cfu mL(-1) in buffer solution. We also tested the specificity of this E. coli detection platform by spiking samples with different species (e.g., Salmonella) to confirm that non-specific binding/detection is negligible. We further demonstrated the proof-of-concept of our approach in a complex food matrix, e.g., fat-free milk, where a similar detection limit of ∼5 to 10 cfu mL(-1) was achieved despite challenges associated with the density of proteins that exist in milk. Our results reveal the promising potential of this cell-phone enabled field-portable and cost-effective E. coli detection platform for e.g., screening of water and food samples even in resource limited environments. The presented platform can also be applicable to other pathogens of interest through the use of different antibodies.

Fig. 1

(A–B) Schematic diagram and picture of the optical attachment for E. coli detection on a cell-phone using the quantum dot based sandwich assay in glass capillary tubes. The entire attachment to the cell-phone weighs ~28 grams (~1 ounce) and has dimensions of ~3.5 × 5.5 × 2.4 cm. This compact and light-weight unit has an imaging field-of-view of 11 mm × 11 mm and can monitor ~10 capillary tubes all in parallel. It can also be repeatedly attached to and detached from the cell-phone body without the need for any fine alignment, making its interface quite easy to operate.

Fig. 2

Dose–response curve for E. coli O157:H7 in 2% gelatine–PBS buffer using the quantum dot based sandwich assay implemented on a cell-phone (Fig. 1B). This response curve provides a linear fit (F = 1.89 × log[E. coli] + 2.36) with R = 0.985. The detection limit is ~5 to 10 cfu mL⁻¹. Inset images show the raw capillary fluorescent images that are captured by our cell-phone fluorescent microscope at different E. coli concentrations. For each measurement point, 3 different samples are used (i.e., n = 3). Standard deviation (σ) for the control sample fluorescent signal level is 0.62.

Fig. 3

Experimental results of our specificity test are summarized. The fluorescent signals from Salmonella contaminated buffer samples at different concentrations are compared against the fluorescent signals from E. coli O157:H7 contaminated buffer samples. As desired, Salmonella contaminated samples did not cause cross-reactivity. For each measurement point, 3 different samples are used (i.e., n = 3). The standard deviation (σ) for the control sample fluorescence signal level is 0.62.

Fig. 4

Dose–response curve for E. coli O157:H7 detection in fat-free milk using the quantum dot based sandwich assay implemented on a cell-phone (Fig. 1B). The curve provides a linear fit (F = 1.58 × log[E. coli] + 2.2) with R = 0.986. The detection limit can be estimated to be ~5 to 10 cfu mL⁻¹ in fat-free milk. Inset images show the raw capillary fluorescent images that are captured by our cell-phone fluorescent microscope at different E. coli concentrations in fat-free milk. For each measurement point, 3 different samples are used (i.e., n = 3). The standard deviation (s) for the control sample fluorescence signal level is 0.4.