Poly(N-3-sulfopropylaniline) (PSPA) is a water-soluble, self-doped conducting polymer belonging to the polyaniline family. It is produced by oxidative polymerization of the monomer N-(3-sulfopropyl)aniline (SPA). PSPA differs from unsubstituted polyaniline (PANI) in that each repeat unit carries a pendant –(CH2)3–SO3H group covalently attached to nitrogen, which acts as an internal dopant (self-doping), conferring water solubility and electroactivity across a wider pH range than PANI. PSPA and its copolymers have been investigated for applications in colorimetric biosensors, amperometric sensors, tissue engineering scaffolds, nanocomposite electrode materials, and aqueous redox flow batteries.
Oxidation states Like unsubstituted polyaniline, PSPA exists in three principal redox forms:
Emeraldine salt (PSPA): the green-colored, partially oxidized, conducting form obtained directly from oxidative polymerization. The polymer is self-doped by the pendant sulfonic acid groups acting as internal dopants. Emeraldine base / sodium salt (PSPANa): the blue-colored, dedoped form obtained by treatment with sodium hydroxide or sodium carbonate. The sulfonic acid groups are converted to sodium sulfonates (–SO3Na) and the backbone acquires a quinoid-rich structure. Four-probe conductivity drops to approximately 6 × 10−5 S/cm. Leucoemeraldine base: the colorless, fully reduced form generated by the action of reducing agents such as ascorbic acid or cysteine on PSPANa. The transition between the emeraldine salt and emeraldine base forms occurs over the pH range 8.6–9.6, producing a visually sharp green-to-blue color change that is exploited in colorimetric sensing.
Synthesis
Monomer The monomer N-(3-sulfopropyl)aniline (SPA) is prepared by ring-opening addition of 1,3-propanesultone to excess aniline.
Chemical oxidative polymerization PSPA is prepared by oxidative polymerization of SPA in aqueous solution using ammonium persulfate (APS) as oxidant, at 0–30 °C, for 2–24 hours. The reaction can be carried out in pure water without added acid, as the reduction of persulfate during polymerization provides sufficient acidification (pH drops to approximately 0.4 in a 0.2 M SPA solution). In the presence of 1 M HCl, additional doping occurs, yielding higher conductivity. The emerald-green polymer is isolated by precipitation into acetone, filtered, washed with acetone, and dried under vacuum at 50–70 °C. An alternative protocol uses 1.2 M HCl as solvent, with a [SPA]:[APS] molar ratio of 1:1, polymerization at 0–5 °C for 5 h followed by overnight aging at 4 °C, and purification by dialysis (1 kDa MWCO membrane, 48 h against pH 3 water) to remove monomer completely, as verified by thin-layer chromatography. Lyophilization yields a green solid (yield ~40%). Molecular weight determined by gel permeation chromatography in DMF: Mw ≈ 8,100 g/mol, Mn ≈ 6,500 g/mol, dispersity ≈ 1.25, corresponding to a degree of polymerization of approximately 31 repeat units. MALDI-TOF analysis shows peaks at mass intervals of ~213 Da, consistent with the repeat unit C9H11NO3S. In samples purified by dialysis, chains of up to 11 repeat units are detected, with the most abundant chain containing 6 units (peak at 1266 Da); from these spectra Mn = 1335 g/mol, Mw = 1404 g/mol, and dispersity Đ = 1.05. Longer chains (7–31 units) are observed in samples isolated by precipitation without dialysis. The sodium salt (PSPANa) is obtained by treating an aqueous PSPA solution with 5 M NaOH solution, precipitating the resulting blue polymer with methanol, and drying under vacuum.
Electrochemical polymerization SPA and its mixtures with aniline can be polymerized electrochemically on electrode surfaces by cyclic voltammetry between −0.2 and 0.85 V (vs. Ag/AgCl) in aqueous sulfuric acid. Sequential deposition (polyaniline first, then SPA) and copolymerization from mixed solutions have both been described.
Properties
Solubility The emeraldine salt (PSPA) is soluble in water, DMSO, NMP, pyridine, and THF, and insoluble in methanol, acetone, and aliphatic or aromatic hydrocarbons. It is also soluble in aqueous base (NaOH, NH4OH), giving red-violet solutions characteristic of the dedoped form. The homopolymer is hygroscopic and retains water from synthesis. In 1.2 M HCl, quantitative determination by UV–visible calibration gives a solubility of 150 mM in monomer units (~3 g/L, corresponding to a theoretical volume-specific capacity of 2010 mAh/L).
Thermal stability Thermogravimetric analysis (TGA, nitrogen atmosphere) shows a first weight loss below 100–120 °C (moisture), a major loss at 200–400 °C attributed to scission of the alkyl sulfonate group and degradation of the backbone, and further decomposition at higher temperatures. The thermal stability follows the order PSPA < SPA < PSPANa; conversion of the acid to the sodium salt improves thermal stability.
Crystallinity Wide-angle X-ray diffraction (WXRD) shows a sharp peak at 2θ ≈ 4–5° (d-spacing ~17–21 Å) attributed to lamellar ordering of polymer chains through ionic interactions between pendant sulfonate anions and imine nitrogen atoms of adjacent chains, resulting in a partially crystalline structure.
Self-assembly in solution Dynamic light scattering (DLS) in aqueous solution reveals nanoaggregates of 8–10 nm with a zeta potential of approximately −20 to −30 mV (PSPA) and −22.5 mV (PSPANa). The amphiphilic character of PSPA (hydrophobic aromatic backbone, hydrophilic sulfonate groups) drives formation of these nanomicellar aggregates in water. The critical micellar concentration (CMC) is approximately 4 × 10−4 M.
Spectroscopic characterization
UV–visible absorption
Upon addition of reducing analytes such as ascorbic acid or cysteine to PSPANa, the absorption at 608–640 nm disappears as the polymer is reduced to the colorless leucoemeraldine form.
NMR spectroscopy The 1H NMR spectrum of PSPA (emeraldine salt, D2O) shows aromatic signals at δ 7.46 (m, 3H) and 7.37 (d, 2H), and aliphatic signals at δ 3.48 (t, 2H, –NH–CH2–), 2.90 (t, 2H, –CH2–SO3H), and 2.00 (t, 2H, central –CH2–). In PSPANa, the aromatic protons shift to δ 6.77 (t, 2H, benzenoid) and 6.40 (d, 4H, quinoid), with loss of the N–H signals and appearance of quinoid ring protons at δ 6.21–6.63 ppm. When APS is used as oxidant, additional singlet peaks at δ 7.00, 7.12, and 7.25 ppm have been assigned to residual NH4+ cations from the oxidant.
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