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

Summary

Here, we present a protocol to detect the adulteration of diesel with kerosene using test strips coated with a fluorescent viscosity probe together with a smartphone-based analysis system.

Abstract

Three fluorescent molecular rotors of 4-dimethylamino-4-nitrostilbene (4-DNS) were investigated for their potential use as viscosity probes to indicate the content of kerosene in diesel/kerosene blends, a wide-spread activity to adulterate fuel. In solvents with low viscosity, the dyes rapidly deactivate via a so-called twisted intramolecular charge transfer state, efficiently quenching the fluorescence. Measurements of diesel/kerosene blends revealed a good linear correlation between the decrease in fluorescence and the increase of the fraction of the less viscous kerosene in diesel/kerosene blends. Immobilization of the hydroxy derivative 4-DNS-OH in cellulose paper yielded test strips that preserve the fluorescent indicator's behavior. Combination of the strips with a reader based on a smartphone and a controlling app allowed to create a simple field test. The method can reliably detect the presence of kerosene in diesel from 7 to 100%, outperforming present standard methods for diesel adulteration.

Introduction

Fuel adulteration is a serious problem in many different parts of the world, simply due to the enormous relevance of fuel as an energy source. Running engines on adulterated fuel reduces their performance, leads to earlier engine failure and entails environmental pollution¹. Increased SO_x emissions occur if diesel is adulterated with kerosene that usually contains a higher amount of sulfur^2,3. Although the problem exists for decades, sustainable fuel management that uncovers such criminal activity at its point of origin is still rare, because simple and reliable tests for fue....

Protocol

1. Fluorescent Dyes (Figure 1A)

1. Purchase commercially available 4-DNS and 4-DNS-OH.
   Note: 4-DNS-COOH is not commercially available and is prepared from 4-DNS-OH as described hereafter.
2. Place 50 mg (0.16 mmol) of 2-[ethyl[4-[2-(4-nitrophenyl)ethenyl]phenyl]amino]ethanol, 2 mg (0.016 mmol) of 4-dimethylaminopyridine and 19.2 mg (0.192 mmol) of succinic anhydride in a 10 mL round bottom flask.
3. Dissolve the reagents in 2 mL of dry dichloromethane under argon atmosphere.
4. Add 11.6 µL (0.08 mmol) of triethylamine and let the mixture react for 20 h.
5. Monitor the reaction by thin layer chromatography until quantita....

Results

The three structures of the two commercial dyes 4-DNS and 4-DNS-OH and the synthesized dye 4-DNS-COOH contain a stilbene core element substituted with a donor (-NR₂) and an acceptor (-NO₂) group at both ends, the central double bond constituting the hinge of the so called 'molecular rotor' (Figure 1A). The structures differ in amino group substitution pattern with short alkyl groups for 4-DNS, two slightly longer groups including an alco.......

Discussion

A fluorescent probe, based on a molecular rotor dye that is sensitive to viscosities in the range of those measured for diesel and its different blends with kerosene, was used to obtain simple and efficient test strips for the detection of diesel fuel adulteration. The emission intensity of 4-DNS at 550 nm in various diesel/kerosene blends correlates with a reduction in viscosity when the proportion of kerosene increases. At a temperature of 24 °C, a nonlinear fluorescence quenching of up to 55% was observed for up .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
4-dimethylamino-4-nitrostilbene (CAS Number: 2844-15-7)	Sigma-Aldrich	39255	4-DNS Dye
2-[ethyl[4-[2-(4-nitrophenyl)ethenyl]phenyl]amino]ethanol (CAS Number: 122258-56-4)	Sigma-Aldrich	518565	4-DNS-OH Dye
Whatman qualitative filter paper, Grade 1	Sigma-Aldrich	Z274852	Test strips support
Whatman application specific filter, activated carbon loaded paper, Grade 72	Sigma-Aldrich	WHA1872047	Fuel pre-treatment filters
Pall reusable in-line filter holders stainless steel, diam. 47 mm	Sigma-Aldrich	Z268453	Holder pre-treatment filters
(3-Aminopropyl)triethoxysilane	Sigma-Aldrich	919-30-2	APTES
4-(Dimethylamino)pyridine	Sigma-Aldrich	1122-58-3	DMAP
Succinic anhydride	Sigma-Aldrich	108-30-5
Triethylamine	Sigma-Aldrich	121-44-8	Et3N
N,N'-dicyclohexylcarbodiimide	Sigma-Aldrich	538-75-0	DCC
Stuart Tube Rotators	Cole-Parmer	SB3	Rotator
FreeCAD	freecadweb.org	-	Freeware - 3D design
Ultimaker Cura	Ultimaker	-	Freeware - 3D printing
Android Studio	Google	-	Freeware - App programming
Renkforce SuperSoft OTG-Mirror Micro-USB Cable 0,15 m	Conrad.de	1359890 - 62	Smartphone setup electronic part
Black Cord Switch 1 x Off / On	Conrad.de	1371835 - 62	Smartphone setup electronic part
Carbon Film Resistor 100 Ω	Conrad.de	1417639 - 62	Smartphone setup electronic part
492 nm blocking edge BrightLine short-pass filter	Semrock	FF01-492/SP-25	Filter excitation
550/49 nm BrightLine single-band bandpass filter	Semrock	FF01-550/49-25	Filter emission
Ø1/2" Unmounted N-BK7 Ground Glass Diffuser, 220 Grit	Thorlabs	DG05-220	Diffuser excitation
LED 465 nm, 9 cd, 20 mA, ±15°, 5 mm clear epoxy	Roithner	RLS-B465	LED excitation