Fluorescent Paper Strips for the Detection of Diesel Adulteration with Smartphone Read-out

The overall goal of these dipstick assays combining a fluorescent viscosity probe and a smartphone readout is to detect diesel adulteration with kerosene and to detect such frauds and avoid economical damage. This method can help users in resource-limited country to carry out rapid tests according to the WHO ASSURED demands The main advantage of this technique is that it gives a qualitative measurement result in less than one minute and can be performed by untrained personnel without specific equipment requirement. Although the method provides a new tool for the detection of fuel adulteration, it can also be used for other onsite measurement applications, like for instance, explosive detection or water analysis.

Prepare one millimolar solutions of the reference dye and dyes 4-DNS, 4-DNS-OH, and 4-DNS-COOH in toluene as described in the text protocol. Cut cellulose strips of 30-by-five millimeters from filter paper. Place approximately 50 of those strips in a sealable five-milliliter vial together with 4.5 milliliters of the desired dye solution.

Shake the strips inside the vial with a vertical rotator for 20 minutes at 30 rpm. Then pour the toluene solution out of the vial. Immediately fill with four milliliters of cyclohexane and rotate for one minute at 30 rpm to wash off excess dyes.

Repeat this washing operation three times. Following wash, dry the obtained test strips on a filter paper for 10 minutes at room temperature before performing sample pretreatment as described in the text protocol. Purchase a standard five-millimeter epoxy LED at 460 nanometers, a 100-ohm resistor, and a USB on-the-go cable with an on-off switch and a micro USB port.

Cut the USB cable on the opposite of the on-the-go side to isolate the red wire powering positive five volts and the black wire corresponding to the ground. Now, cut the black wire of the USB cable and solder the 100-ohm resistor on the back of the switch. Solder the LED anode to the positive five-volt red wire and the LED cathode to the ground black wire.

Purchase a diffuser and two filters for the LED and the camera, typically a short-pass filter for the excitation channel and a band-pass filter for the emission collection. 3D print a smartphone case that fits on the smartphone and integrates the different optical parts consisting of a black chamber as described in the text protocol. Also, 3D print a strip holder to hold a reference and a test strip.

Implement the excitation channel by placing the LED, the diffuser, and the filter to illuminate the paper strips at an angle of 60 degrees. Implement the reading channel by placing the filter in front of the smartphone CMOS camera. Select the adequate calibration file from the software memory by clicking on the menu button in the upper right corner of the software window.

Dip the test strip into the diesel sample for a couple of seconds by holding the test strip with tweezers. Remove excess fuel by padding with a drying paper. Place the test strip inside the strip holder beside the reference strip and introduce the holder into the smartphone case.

An image of the strip's fluorescence is then immediately displayed on the smartphone's screen. Press the shoot button to record the fluorescence intensities of test and reference strips. The degree of adulteration is immediately calculated by the internal algorithm and displayed on the screen.

To obtain test strips coded with a fluorescence molecular rotor as a viscosity probe, three types of dyes were coded on paper. One candidate, 4-DNS-OH, was found to be the most suitable probe for detecting diesel adulteration with kerosene. The fluorescence of the strips was determined with a smartphone camera after insertion of the strips into a 3D-printed case.

The case integrated the strip holder and all necessary optical elements, such as an LED powered directly by the smartphone USB port, filters, and a diffuser. The assay procedure was kept as simple as possible with only a couple of steps, dipping, placing the strip in the holder, starting the LED, and positioning the holder in the case, followed by measurement and analysis. The analysis software averaged all the RGB values of the pixels in predefined spatial areas corresponding to the strips and converted them to fluorescence intensities.

Once mastered, this technique can be done within one or two minutes. After watching this video, you should have a good understanding of how such cost-effective, precise, and rapid methods for fuel adulteration should work. It's also interesting forensic solution, for instance, if an untrained authority personnel or consumers should uncover fraud.

While attempting this procedure, it's important to remember to choose the adequate calibration files specially when working with any diesel samples of different grades, as they can interact specifically on the strip. Following this procedure, other reference or laboratory methods like GC-FID can be performed in order to validate positive results observed on-field during a screening campaign. So, this technique can pave the way for other researchers to combine fluorescent dipstick assays with embedded smartphone readout systems to provide valuable tools for onsite detection like pollutants in water or other forensic applications.

Working with smartphone CMOS alt-detector requires some precautions concerning the outer-wide valence algorithm that is present in most of the devices. Measuring the fluorescence of a reference strip besides the test strip permits correction of such deviation.