A luminol–HAuCl4–pepsin (Pep) flow injection-chemiluminescence (FI-CL) system was explored to determine captopril (CAP) based on the CL intensity inhibition effect and applied to study CAP pharmacokinetics in rabbits with microdialysis. HAuCl4 and pepsin (Pep) could significantly enhance the luminol chemiluminescence (CL) intensity. It was found that sub-nanometre Au nanoclusters (AuNCs) were generated in the luminol–HAuCl4–Pep reaction solution. A possible mechanism for AuNCs generation is given. By means of the FI-CL and molecular docking (MD) methods, the Pep–CAP interaction was systematically studied. The results showed that CAP might enter into Pep active site Asp32 with the binding constant (K) 1.7 × 106 L mol−1, which could effectively inhibit the CL intensity. The CL intensity could be remarkably inhibited by CAP and the decrement of CL intensity was linearly correlated to the logarithm of CAP concentration in the range of 3.0 pmol L−1 to 0.1 μmol L−1, with a detection limit of 1.0 pmol L−1 (3σ). This proposed approach was successfully applied to determine CAP in rabbit’s blood during the 16 h after intragastric administration with an elimination ratio of 45.9% and recovery ratios from 89.0% to 112.0%. The pharmacokinetic results showed that CAP could be rapidly absorbed into blood with a peak concentration (Cmax) of 9.63 ± 1.45 μg mL−1 at a maximum peak time (Tmax) of 0.75 ± 0.08 h; the elimination half-life of 3.19 ± 0.13 h and the elimination rate constant of 7.27 ± 0.41 L g−1 h−1 in rabbits were derived, respectively.

Size and shape controlled Au nanomaterials (AuNMs) were generated in different alkaline luminol–pepsin (Pep) chemiluminescence (CL) reaction solutions based on the photochemical induced effect of luminol. These findings showed that luminol photochemical intensity could be used as a major control parameter for metal NMs growth.

Luminescently probed by luminol, the interaction behaviors of bovine serum albumin (BSA) with different antibiotics (macrolides, tetracyclines and sulfonamides) at picomolar levels were investigated using flow injection chemiluminescence (FI-CL) analysis. It was found that BSA and antibiotics formed complexes with the binding ratio of 1:1, with the binding constants K within the range of 103 to 105 L mol−1 generally following the order of macrolides < tetracyclines < sulfonamides. Results showed that the ester groups of –OOCC3H7 or –OOC(CH2)2COOC2H5 in macrolides led to increased K by factors of 3.1–42.8 times, –OH or –Cl in tetracyclines increased K by 1.1–1.3 times, and the additional –C(NH)NH2 in sulfonamides increased K about 1.3 times. The thermodynamic parameters demonstrated that the BSA–antibiotic binding process should be spontaneous mainly through hydrophobic interaction for macrolides, hydrogen bonding and van der Waals force for tetracyclines, and electrostatic interaction for sulfonamides. The further antibiotics to BSA molecular docking study revealed that the pocket at subdommain IIA of BSA should be the principle binding site for antibiotics, showing that the interaction parameters including K and ΔG agreed well with the results from the proposed FI-CL analysis. The interesting phenomenon that the logK had good linear correlation to molecular volume MV, molar refractivity MR, polarizability POL and partition coefficient logP of antibiotics was also observed.

The interaction behavior of lysozyme with trivalent lanthanide ions (LnIII) was investigated by flow injection chemiluminescence (FI-CL) analysis. It was found that lysozyme with LnIII could form a 1:1 association complex that remarkably quenched CL intensity from the luminol–lysozyme reaction, and the quantitative correlation equation of CL intensity decrements versus the logarithm of the concentration of LnIII ΔI = AlnCLn + B was established. The results showed the sensitive factor A varied as atomic number Z increased monotonically, and ALL of light lanthanides (LL) from LaIII to EuIII was less than AHL of heavy lanthanides (HL) from GdIII to LuIII. The relationships of A vs. Z plot for LL and HL with a GdIII break at f7 were clearly presented, and the correlations of A with physical parameters (γ±, ΔHhyd and Eo) of LnIII were discussed. Using a homemade FI-CL model, the binding constants (K = 105 to 106 level) and binding sites (n = 1.0) were obtained, and the binding affinity abilities increased with increasing Z along the LnIII series. The thermodynamic parameters of lysozyme with LnIII were obtained, and the results showed that the reaction was a spontaneous process by electrostatic interaction. The possible mechanism and application of the luminol–lysozyme–LnIII CL system were given.

•The FL intensity of luminol was quenched by myoglobin and other proteins.•The fluorescence quenching mechanism was studied.•A mathematical equation of lg(F0 − F)/F = 1/nlg[P] + 1/nlgKa was first constructed.•The interaction parameters (Ka and n) between luminol and proteins were obtained.In this paper, a new mathematical equation of lg(F0 − F)/F = 1/nlg[P] + 1/nlgKa, which was used to obtain interaction parameters (the binding constant Ka and the number of binding sites n) between the protein and the small molecule ligand by using the ligand as a fluorescence (FL) probe, was constructed for the first time. The interaction parameters between myoglobin, catalase, lysozyme, bovine serum albumin (BSA) and luminol were obtained by this equation with luminol used as a FL probe, showing that the binding constants Ka were 8.78 × 105, 4.47 × 105, 4.21 × 104 and 3.95 × 104 respectively, and the number of binding sites n approximately equaled to 1.0 for myoglobin, catalase, and 2.0 for lysozyme, BSA. The interactions of ferritin, ovalbumin, aldolase, chymotrypsinogen and ribonuclease with luminol were also studied by this method. The binding constants Ka were at 104–105 level, and the number of binding sites n mostly approximately equaled to 2.0. The binding ability of luminol to the studied proteins followed the pattern: myoglobin > aldolase > ferritin > ovalbumin > catalase > ribonuclease > lysozyme > BSA > chymotrypsinoge.

In this paper, the luminescence behavior of bovine serum albumin (BSA) and luminol was first studied by flow injection chemiluminescence (CL). It was found that the hyperchromic effect of luminol in the presence of BSA led to the acceleration of the electrons transferring rate of excited 3-aminophthalate, which greatly enhanced the CL intensity of luminol/dissolved oxygen reaction. The increments of CL intensity were proportional to the concentrations of BSA with a linear range from 0.01 to 7 nmol L−1. It was also found that azithromycin could inhibit the CL intensity of luminol/BSA reaction. The decrements of CL intensity were logarithm over the concentrations of azithromycin ranging from 0.1 to 700 ng mL−1. At a flow rate of 2.0 mL min−1, a complete analytical process, which included sampling and washing, could be performed within 30 s with relative standard deviations of less than 3.1%. This proposed method was successfully applied in assaying azithromycin in pharmaceutical and human serum samples with recoveries from 91.0 to 104.3%. The possible luminescence mechanism of luminol/BSA/azithromycin reaction was discussed in detail by CL, UV and fluorescence methods.Graphical abstractHighlights► BSA can catalyze and enhance the CL intensity of luminol. ► AZM can quench the signal of luminol-BSA CL system.

The photochemical reaction mechanism of lysozyme with cephalosporin analogues was investigated with luminol used as a luminescence probe by flow injection chemiluminescence. It was found that Glu35 and Asp52 of lysozyme accelerated the rate of excited 3-aminophthalate electrons transferring and enhanced the chemiluminescence signal of luminol, producing steady-state chemiluminescence in the flow injection system with relative standard deviations less than 3.0%. It was also found that cephalosporin analogues could enter into the site of Trp62 in lysozyme forming 1:1 complex which leads to a conformational change of lysozyme, giving the effect of chemiluminescence quenching from luminol–lysozyme. Based on the photochemical behavior of luminol/lysozyme and cephalosporin, a model of lysozyme–cephalosporin interaction, lg[(I0 – I)/I]=lgKD + nlg[D], was established. Using the proposed model, the interaction parameters and the binding ability of lysozyme with cephalosporin were successfully obtained, and the results agreed very well with the results obtained by fluorescence.

The determination of Sudan I in contaminated hot chilli products by luminol–myoglobin chemiluminescence was proposed. It was found that the Sudan dyes (Sudan I, II, III and IV) could bind to myoglobin and remarkably enhance the chemiluminescence signal from luminol–myoglobin system, and the increment of chemiluminescence intensity was proportional to the concentration of Sudan I, II, III and IV in the range of 0.1–300 pg mL−1, 5.0–1000 pg mL−1, 10.0–1000 pg mL−1 and 1.0–300 pg mL−1, respectively. At the flow rate of 2.0 mL min−1, a typical analytical procedure for Sudan dyes including sampling and washing was finished within 0.5 min with the relative standard deviation less than 3.0%. The present CL method was successfully applied to the determination of Sudan I in contaminated soy sauces and agreed well with HPLC data. The possible mechanism of the CL reaction was given.

The determination of Sudan I in contaminated hot chilli products by luminol–myoglobin chemiluminescence was proposed. It was found that the Sudan dyes (Sudan I, II, III and IV) could bind to myoglobin and remarkably enhance the chemiluminescence signal from luminol–myoglobin system, and the increment of chemiluminescence intensity was proportional to the concentration of Sudan I, II, III and IV in the range of 0.1–300 pg mL−1, 5.0–1000 pg mL−1, 10.0–1000 pg mL−1 and 1.0–300 pg mL−1, respectively. At the flow rate of 2.0 mL min−1, a typical analytical procedure for Sudan dyes including sampling and washing was finished within 0.5 min with the relative standard deviation less than 3.0%. The present CL method was successfully applied to the determination of Sudan I in contaminated soy sauces and agreed well with HPLC data. The possible mechanism of the CL reaction was given.