Conducting Polymer Composite Snaps Dark to Reveal Toxic Chromium and Iron in Water

A water-soluble composite of polyaniline and sulfonated phenol formaldehyde resin has emerged as a strikingly sensitive fluorescent probe for two of the most problematic contaminants in drinking water: hexavalent chromium and ferric iron. In a study published in Polymer Bulletin, Murugan Rajeswari and Konda Kannan Satheesh Kumar of Gandhigram Rural Institute in Tamil Nadu, India, describe how a simple in situ oxidative polymerisation route in aqueous solution produces a family of PANI–SPF composites, designated PANI–SPF 1 through 6, whose fluorescence collapses in the presence of vanishingly small amounts of these metal ions. The detection limits they report, 0.61 nanomolar for Cr6+ and 0.68 nanomolar for Fe3+ using the best composite in the series, place the material among the most sensitive polymer-based turn-off chemosensors described for these targets, and the same material doubles as a potent antibacterial agent.

The significance of the target analytes is difficult to overstate. Hexavalent chromium, released into surface and groundwater by leather tanning, electroplating, pigment manufacturing and other industries, is a recognised carcinogen whose oxyanion species readily cross cell membranes and wreak oxidative damage on DNA and proteins. Ferric iron, while an essential nutrient, becomes a contaminant of its own when excessive concentrations stain water, promote the growth of iron bacteria in distribution systems, and, in biological contexts, catalyse the formation of hydroxyl radicals through Fenton chemistry, a process implicated in neurodegenerative disease pathology. Conventional detection of these ions relies on instrumental techniques such as atomic absorption spectroscopy or inductively coupled plasma mass spectrometry, which are accurate but expensive, laboratory-bound and poorly suited to real-time field screening. Fluorescent chemosensors offer an attractive alternative because the signal can be read with a simple spectrometer or even the naked eye under ultraviolet light.

The Indian team’s material is a marriage of two well-understood components. Polyaniline is the archetypal conducting polymer, prized for its reversible acid-base doping chemistry, environmental stability and intrinsic fluorescence, while sulfonated phenol formaldehyde is a water-soluble anionic resin whose sulfonate groups act as dopant and steric stabiliser at once. By polymerising aniline in the presence of varying amounts of the sulfonated resin, the researchers tuned the composition of the resulting composites, and the interplay between the conducting polymer backbone and the sulfonated matrix proved decisive for every property they measured. The composites were characterised exhaustively by Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis and thermogravimetric analysis, confirming successful composite formation and revealing how the resin content reshapes the polymer’s morphology and thermal behaviour.

Electrical conductivity measurements singled out one member of the family. The PANI–SPF 3 composite, with an intermediate resin loading, achieved a conductivity of 6.31 multiplied by 10 to the power of minus 3 siemens per centimetre, the highest in the series. This optimum reflects a classic doping balance: enough sulfonated resin to protonate and order the polyaniline chains into the electrically favourable emeraldine salt form, but not so much that the insulating resin dilutes the conductive pathways. The same intermediate composition also turned out to be the star performer optically and biologically, a coincidence the authors attribute to the density of accessible binding sites and the quality of the photophysical pathway within the material.

The sensing experiments were carried out in a dimethyl sulfoxide and water mixture, nine parts to one by volume, using ultraviolet-visible absorption and fluorescence spectroscopy. In the pristine state, the PANI–SPF 3 composite emits a measurable fluorescence signal. When aliquots of Cr6+ or Fe3+ are added, that emission is progressively extinguished, and the degree of quenching follows a linear Stern-Volmer relationship across the concentration range studied. Linearity in Stern-Volmer plots is the fingerprint of a well-behaved quenching mechanism, and it is what allows the intensity ratio to be converted directly into an ion concentration. The nanomolar detection limits derived from these plots represent an extraordinary amplification factor: a few hundred millionths of a gram of toxic metal per litre is enough to visibly darken the sensor’s output.

Several physical mechanisms can drive fluorescence turn-off in conducting polymer composites, and the authors discuss the likely contributors in terms of complex formation between the metal ions and the electron-rich nitrogen and sulfonate sites on the composite. Divalent and trivalent metal cations can coordinate to these sites, opening non-radiative decay channels or promoting photoinduced electron transfer that drains the excited-state energy before a photon can be emitted. Inner filter effects, in which the metal ion absorbs the excitation or emission light, may also contribute. The density functional theory calculations reported in the paper lend computational weight to this picture, modelling the complexes formed between the composite’s constituent units and the metal ions and clarifying why Cr6+ and Fe3+ quench so efficiently. The same computational framework was extended to rationalise the observed antimicrobial effects, connecting the electronic structure of the material to its biological activity.

The antimicrobial results are arguably the study’s most surprising dimension. All six composites were screened against Escherichia coli, a Gram-negative bacterium, and Staphylococcus aureus, a Gram-positive one, and PANI–SPF 3 again led the pack, producing zones of inhibition that the authors describe as notable in comparison with Gentamicin, a clinical antibiotic used as the reference standard. Conducting polymers have a growing reputation as antimicrobial agents, with proposed mechanisms including disruption of bacterial membrane potentials, electrostatic adhesion of the positively charged polymer backbone to negatively charged cell surfaces, and interference with electron transport across the membrane. The sulfonated resin component may add further activity by chelating essential metal ions from the microbial environment. A dual-function material that both reports contamination and suppresses microbial growth is an unusual and appealing combination for water treatment and packaging applications.

Safety is the perennial question for any material proposed for contact with drinking water, and the team addressed it directly. An MTT cytotoxicity assay, which measures the metabolic activity of cultured cells as a proxy for viability, showed that the PANI–SPF 3 composite is non-cytotoxic at concentrations up to 500 micrograms per millilitre. That margin provides a measure of reassurance for applications in which the sensor might contact potable water or come into contact with biological surfaces, although the authors are careful to frame this as an early-stage result rather than a regulatory clearance. The work also builds on the group’s own recent lineage of sulfonated-resin composites, including polyaniline and polypyrrole paired with sulfonated naphthalene formaldehyde and sulfonated melamine formaldehyde systems, which targeted chromium speciation, lead and mercury sensing, antifungal action and corrosion protection, suggesting a versatile materials platform rather than a one-off discovery.

What distinguishes this study within the crowded field of fluorescent heavy-metal sensors is the combination of sensitivity, simplicity and multifunctionality achieved with inexpensive, water-processable ingredients. Many high-performance chemosensors rely on elaborate organic syntheses, rare-earth dopants or quantum dots that complicate scale-up and raise their own toxicity questions. Here, the synthesis is a one-pot oxidative polymerisation in water, the characterisation is standard, and the resulting material conducts electricity, senses two priority contaminants at nanomolar levels, and kills bacteria on contact. The linear Stern-Volmer response means quantification requires nothing more exotic than a fluorescence spectrometer and a calibration curve, and the turn-off format, while inherently susceptible to false positives from other quenchers, benefits from the strong selectivity implied by the complexation chemistry and the DFT analysis.

The road from laboratory cuvette to field deployment remains long, as it always does for fluorescent sensors, with real water matrices, competing ions, pH variation and sensor regeneration all demanding attention. But the PANI–SPF work offers a compelling proof of concept that the humblest of industrial polymers, phenol formaldehyde chemistry dating back more than a century, can be re-engineered through sulfonation and combination with a conducting polymer into a twenty-first-century analytical tool. If subsequent studies confirm the selectivity in genuine environmental samples and establish long-term stability, composites of this kind could find their way into portable water-quality kits, smart packaging indicators and antimicrobial filtration membranes, turning the glow of a polymer film into an early-warning system for the contaminants that most threaten the world’s drinking water.

Subject of Research: Fluorescent PANI–SPF polymer composite chemosensor for detecting Cr6+ and Fe3+ ions in water with antimicrobial and DFT analysis

Article Title: Development of a polyaniline – sulfonated phenol formaldehyde (PANI–SPF) composite as a fluorescence ‘Turn-off’ chemosensor for Cr6+ and Fe3+ ions, antimicrobial application and DFT investigation

Article References: Rajeswari, M., & Kumar, K. K. S. (2026). Development of a polyaniline – sulfonated phenol formaldehyde (PANI–SPF) composite as a fluorescence ‘Turn-off’ chemosensor for Cr6+ and Fe3+ ions, antimicrobial application and DFT investigation. Polymer Bulletin, 83(12), Article 679. https://doi.org/10.1007/s00289-026-06738-2

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06738-2

Keywords: polyaniline, sulfonated phenol formaldehyde, fluorescence chemosensor, hexavalent chromium, ferric iron, heavy metal detection, Stern-Volmer quenching, antimicrobial activity, DFT, water quality, conducting polymer, limit of detection