What if a single three-microliter water droplet could reveal toxic hexavalent chromium without immersing a probe or using a large instrument? This paper builds that idea around a glowing europium complex and a superhydrophobic membrane.

Chromium-contaminated liquid waste is a major concern in industries including rubber, leather, paper, tanning, and sanitary landfills. Chromium compounds containing hexavalent chromium are bioaccumulative and highly toxic upon consumption. The World Health Organization has set the permissible Cr(VI) concentration level in drinking water at 50 micrograms per liter, creating a clear need for simple, inexpensive, and efficient chromium sensing technologies.

Existing fluorescence sensors based on quantum dots and organic dyes can suffer from low sensitivity, short fluorescence lifetimes, broad emission bands, and photobleaching. Lanthanide-complex sensors using the inner filter effect overcame these limitations, but remained vulnerable to water.

Conventional detection methods include surface-enhanced Raman spectroscopy and high-performance liquid chromatography coupled with ultraviolet-visible determination. These methodologies are limited by high cost, complicated operation, and time consumed.

Luminescent sensing offers high sensibility, short response time, easy manipulation, and low cost compared with conventional instrumental methods. The specific gap was that non-contact fluorescence sensing based on the inner filter effect had never been investigated with lanthanide complexes for Cr(VI) detection.

The proposed approach is a lanthanide-complex-based non-contact fluorescence sensing approach coupled with the inner filter effect for Cr(VI) detection in aqueous liquids with high sensitivity and high selectivity. The sensor uses a nanofibrous membrane electrospun from polyvinylidene fluoride, silica particles, and Eu(TTA)3Phen complex, with TTA meaning 2-thenoyltrifluoroacetone and Phen meaning 1,10-phenanthroline.

The Eu3+ complex was chosen because its spectrum overlaps with Cr(VI), while silica particles and fluor alkyl chemistry together produce superhydrophobicity, meaning extreme water repellency. Superhydrophobicity makes water bead up on the sensor, allowing Cr(VI) detection with three-microliter droplets.

The sensor demonstrated detection as low as 0.44 micromolar, or 45.76 micrograms per liter, with high selectivity against metal cations and anions in water. Figure one schematically shows fabrication of the Cr(VI) sensor by electrospinning a blend of EuC, PVDF, and silica particles into a nanofibrous membrane, followed by treatment with PFOTS.

It also illustrates the sensing configuration: a single aqueous droplet on the membrane is excited at three hundred sixty-five nanometers, and the emitted fluorescence at six hundred twelve nanometers is collected. This matters because the figure connects the membrane’s material design with the optical readout used for Cr(VI) detection.

Cr(VI) sensing with aqueous droplets was conducted on FM 5, the Cr(VI) sensor, using fluorescence spectra obtained from microspectroscopy. Multiple stock solutions covered a wide range of Cr(VI) concentrations by diluting a 0.1 molar dichromate solution prepared using potassium dichromate.

A three-microliter droplet was placed over the sensor, excited using a 365-nanometer xenon light source, and measured for its fluorescence response. Each concentration was tested at least two times with identical parameters at room temperature.

Selectivity was evaluated with 0.1 molar stock solutions containing a series of metal cations, including barium, calcium, potassium, magnesium, manganese, and sodium. The sensor consists of a nanofibrous membrane electrospun from polyvinylidene fluoride, silica particles, and Eu(TTA)3Phen complex.

To test the role of each component, five membranes were fabricated. The sequence ran from Eu3+ complex only, to PVDF only, PVDF plus Eu3+ complex, PVDF plus Eu3+ complex plus silica particles, and finally FM 4 modified with PFOTS to create FM 5, the Cr(VI) sensor. Scanning electron microscopy showed a nanofibrous morphology for FM 2, FM 3, FM 4, and FM 5, while silica particles in FM 4 and FM 5 provided additional texture.

After PFOTS modification, FM 5 displayed superhydrophobicity from the silica texture and the low solid surface energy imparted by PFOTS. Its water static contact angle was 170 degrees, and droplets rolled off at a five-degree tilt angle. Figure Two shows the Cr(VI) sensor as a dense, interconnected nanofibrous membrane in SEM images at progressively higher magnification, with scale bars of fifty micrometers, five micrometers, and two micrometers.

The final panel shows a three microliter water droplet beading on the surface, with a contact angle of one hundred seventy degrees. Together, these images document both the electrospun fiber structure and the sensor’s strongly water-repellent surface, properties relevant to analyzing fluorescence responses from aqueous droplets.

Figure 3 presents XPS characterization of FM 3 and FM 5, the latter being the Cr(VI) sensor. The survey spectra identify surface elements including fluorine, carbon, oxygen, nitrogen, and europium, while the high-resolution panels resolve chemical environments such as C–F, C–O, C–N, C=O, and Si–O.

These signatures support the presence of the PVDF matrix, silica particles, and the Eu three-plus complex in the nanofibrous membrane. The photophysical properties of the Cr(VI) sensor, FM 5, were characterized using fluorescence spectra. Samples containing the Eu3+ complex showed an excitation peak between 300 and 400 nanometers and a sharp emission peak at 612 nanometers.

The 612-nanometer emission corresponds to Eu3+ energy-level transitions, with the listed transitions occurring at 579, 590, 612, 651, and 702 nanometers. The fluorescence intensity depends on energy transfer resulting from excitation in the Eu3+ complex, and that intensity is a critical parameter determining sensitivity.

Figure four compares the Eu³⁺-containing samples through emission, excitation, and time-resolved fluorescence measurements. The emission spectra show the characteristic sharp Eu³⁺ peak near six hundred twelve nanometers, while the excitation spectra contain features in the roughly three-hundred-to-four-hundred-nanometer region.

Panel c tracks fluorescence counts over time in microseconds, supporting the authors’ discussion of sample-dependent lifetimes and the reduced photophysical response of the Cr(VI) sensor, FM five. Sensitivity was investigated by detecting Cr(VI) in aqueous droplets across concentrations from one to 80 micromolar.

Figure 5a shows overlap between the sensor excitation spectrum and the Cr(VI) absorption spectrum. Increasing Cr(VI) concentration produced increasing absorption intensities in the droplets. As a consequence, the sensor's emission intensities were suppressed with increasing Cr(VI) concentration; this is fluorescence quenching.

At high Cr(VI) concentrations, Figure 5b shows dramatic suppression of emission, with fluorescence completely quenched at 200 micromolar. Figure five links Cr(VI) absorption to sensor response: as Cr(VI) concentration increases from one to eighty micromolar, absorption grows and fluorescence is quenched, with a linear Stern–Volmer relationship from one to sixty micromolar.

The selectivity panels show a distinct absorption band between three hundred ten and four hundred five nanometers for Cr(VI), while the tested cations and anions lack that band and produce no discernible fluorescence quenching, with F-zero over F equal to one. The quenching efficiency was analyzed using the Stern–Volmer equation, F zero divided by F equals one plus K sub S V times C.

F zero is the fluorescence intensity without a droplet, F is the intensity with a droplet, C is the Cr(VI) concentration, and K sub S V is the Stern–Volmer constant. F zero divided by F increased linearly with Cr(VI) concentration from one to 60 micromolar, with a correlation coefficient of R squared equal to 0.95544.

The limit of detection was calculated using three sigma divided by K sub S V, where sigma is the standard deviation. The resulting limit of detection was 0.44 micromolar, or 45.76 micrograms per liter. Selectivity was tested with metal cations—barium, calcium, potassium, magnesium, manganese, and sodium—and anions including fluoride, bromide, chloride, iodide, bisulfate, and acetate in aqueous droplets.

The absorption band between 310 and 405 nanometers appeared only with Cr(VI), not with the tested metal cations and anions. Spectral overlap between the Eu3+ complex excitation and Cr(VI) absorption produced fluorescence quenching through the inner filter effect.

The proposed sensing mechanism could be static quenching because the fluorescence decay rates showed no effect for the Cr(VI) fluorescence sensor. The tested cations and anions produced no discernible quenching effect even at 0.1 molar, demonstrating high selectivity in these droplets.

The sensor combines europium-complex fluorescence, inner filter effect quenching, and superhydrophobic droplet handling to detect Cr(VI) down to 0.44 micromolar, while showing selectivity against tested ions in water.