Facile, sensitive and selective fluorescence turn-on detection of HSA/BSA in aqueous solution utilizing 2,4-dihydroxyl-3-iodo salicylaldehyde azine

A novel fluorescence turn-on detection method of human serum albumin (HSA) and bovine serum albumin (BSA) in aqueous solution is investigated using 2,4-dihydroxyl-3-iodo salicylaldehyde azine (DISA). Upon the addition of DISA to HSA/BSA solution, a fluorescence turn-on effect at 529 nm can be observed with a large stokes shift of ∼129 nm based on hydrophobic binding-mode between protein and dye. Under the optimal condition, the linear ranges of fluorescence intensity for HSA and BSA are 0.1-30 μg mL⁻¹ with the relative correlation coefficient of R² = 0.991 (n = 10) and 0.3-50 μg mL⁻¹ with R² = 0.997 (n = 10); and the detection limits for HSA and BSA based on IUPAC (C_DL = 3S_b/m) are 20 ng mL⁻¹ and 50 ng mL⁻¹, respectively.     

Spectrofluorimetric determination of human serum albumin using a doxycycline-europium probe

A new spectrofluorimetric method is proposed for determination of human serum albumin (HSA) with the limit of detection at ng levels. Using doxycycline (DC)-europium (Eu³⁺) as a fluorescent probe, in a buffer solution of pH 10.2, HSA can remarkably enhance the fluorescence intensity of the DC-Eu³⁺ complex at 612 nm and the enhanced fluorescence intensity of Eu³⁺ is proportional to the concentration of HSA. Optimum conditions for the determination of HSA are also investigated. The linear ranges for HSA are 0-9.2 and 9.2-34.5 μg ml⁻¹ with limits of detection of 64 and 115 ng ml⁻¹, respectively. This method is simple, practical and relatively free of interference from coexisting substances, as well as much more sensitive than most of the existing assays. The determination results for human serum and urine samples are identical to those by the AOAO method, with relative standard deviations of five determinations of 1.1-3.6%. By the Rosenthal graphic method, the binding number and association constant of human serum albumin with the probe are 1.8 and 3.71×10⁵ l mol⁻¹, respectively.     

Determination of gamma-globulin at nanogram levels by its enhancement effect on the resonance light scattering of functionalized HgS nanoparticles

A novel assay of γ-globulin (γ-IgG) with a sensitivity at the nanogram level is proposed based on the measurement of enhanced resonance light-scattering (RLS) signals resulting from the interaction of functionalized nano-HgS with γ-globulin. At pH 5.03, the RLS signals of functionalized nano-HgS were greatly enhanced by γ-globulin in the region of 200-700 nm characterized by the peak around 362 nm. Linear relationship can be established between the enhanced RLS intensity and γ-globulin concentration in the range of 10-140 ng ml⁻¹. The limit of detection is 2.71 ng ml⁻¹. Based on this, a new direct quantitative determination method for γ-globulin in blood serum samples without separation of human serum albumin (HSA) is established. The contents of γ-IgG in blood serum samples were determined with recovery of 95.7-102.5% and R.S.D. of 1.6-2.4%. This method is proved to be very sensitive, rapid, simple and tolerance of most interfering substances.     

Rapid and sensitive determination of protein by light-scattering technique with Eriochrome Blue Black R

Based on the interaction between Eriochrome Blue Black R (EBBR) and proteins, which causes a strong light-scattering signal with the maximum scattering peak located at 398 nm, a simple, rapid, sensitive and selective method is developed for the determination of proteins by the light-scattering technique using a common spectrofluoremeter. Under proper experimental conditions, the protein determination can be performed in the range of 0.1-25, 0.1-20 and 0.25-25 μg ml⁻¹ for bovine serum albumin (BSA), human serum albumin (HSA) and human immunoglobulin G (IgG), respectively. The detection limit, calculated as 3 times the S.D. of nine blank measurements, are 33 μg l⁻¹ for BSA, 25 μg l⁻¹ for HSA and 38 μg l⁻¹ for IgG. Moreover, there is no significant difference among the scattering signals yielded by HSA, IgG and BSA, and almost no interference of many amino acids and metal ions. The method has been satisfactorily applied to the direct determination of the total protein in human serum, saliva and urine samples. The results obtained from the studies on the binding characteristics of EBBR to BSA indicated that an electrostatic force existed in the binding system, and the binding constant (K) and the number of the binding sites (n) at 25 °C are 1.69×10⁵ l mol⁻¹ and 0.946, respectively.     

Determination of the antidepressant paroxetine and its three main metabolites in human plasma by liquid chromatography with fluorescence detection

A high-performance liquid chromatographic method has been developed for the determination in human plasma of the specific serotonin reuptake inhibitor (SSRI) antidepressant paroxetine and its three main metabolites (M1, M2, M3). Fluorescence detection was used, exciting at λ = 294 nm and monitoring emission at λ = 330 nm for paroxetine (λ_exc = 280 nm, λ_em = 330 nm for M1 and M2; λ_exc = 268 nm, λ_em = 290 nm for M3). Separation was obtained on a reversed-phase C18 column using a mobile phase composed of 66.7% aqueous phosphate at pH 2.5 and 33.3% acetonitrile. Imipramine (λ_exc = 252 nm, λ_em = 390 nm) was used as the internal standard. A careful pre-treatment of plasma samples was developed, using solid-phase extraction with C8 cartridges (50 mg, 1 mL). The calibration curves were linear over a working range of 2.5-100 ng mL⁻¹ for paroxetine and of 5-100 ng mL⁻¹ for all metabolites. The limit of detection (LOD) was 1.2 ng mL⁻¹ for PRX and 2.0 ng mL⁻¹ for the metabolites. The method was applied with success to plasma samples from depressed patients undergoing treatment with paroxetine. Hence, the method seems to be suitable for the therapeutic drug monitoring of paroxetine and its main metabolites in depressed patients’ plasma.     

A novel nonenzymatic fluorescent sensor for glucose based on silica nanoparticles doped with europium coordination compound

Amino-functionalized luminescent silica nanoparticles (LSNPs) doped with the europium(III) mixed complex, Eu(TTA)₃phen with 2-thenoyltrifluoroacetone (TTA) and 1,10-phenanthroline(phen) were synthesized successfully using an revised Stöber method. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR), and fluorescence spectroscopy were performed for characterizing the synthesized nanoparticles. In the presence of glucose, the fluorescence intensity of the amino-functionalized LSNPs was enhanced due to the enhanced fluorescence resonance energy transfer. Based on fluorescence-enhancing effect, a simple and sensitive method for the determination of glucose was proposed. Under the optimized experimental conditions, the enhanced fluorescence intensity ratio (ΔF/F₀) was linear with the concentration of glucose (c) in the range of 0.0-180 μg ml⁻¹ with a detection limit of 0.8 μg ml⁻¹ (S/N = 3). The R.S.D. values were 0.33% and 0.37% at the levels of 22.5 and 100 μg ml⁻¹, respectively. The proposed method was applied to the determination of glucose in synthetic samples with satisfactory results. The proposed method was also performed to the analysis of blood glucose in human serum samples and the results were in good agreement with clinical data provided by the hospital, which indicates that the method presented here is not only simple, sensitive, but also reliable and suitable for practical applications.     

Successive determination of urinary protein and glucose using spectrophotometric sequential injection method

A new sequential injection (SI) system with spectrophotometric detections has been developed for successive determination of protein and glucose. The protein assay is based on ion-association of protein with tetrabromophenolphthalein ethyl ester (TBPE) in the presence of Triton X-100 at pH 3.2. The blue product is monitored for absorbance at 607 nm. For glucose, hydrogen peroxide, generated by the oxidation of glucose in the presence of glucose oxidase immobilized on glass beads packed in a minicolumn, is monitored using iron-catalyzed oxidation reaction of p-anisidine to form a red colored product (520 nm). The SI procedure takes advantage in performing the protein assay during the incubation period for glucose oxidation. Linear ranges were up to 10 mg dL⁻¹ human serum albumin (HSA) with a limit of detection (LOD) (3σ) of 0.3 mg dL⁻¹, and up to 12.5 mg dL⁻¹ glucose with LOD of 0.08 mg dL⁻¹. R.S.D.s (n = 11) were 2.7% and 2.5% (for 1 mg dL⁻¹ and 5 mg dL⁻¹ HSA) and 1.4% (9 mg dL⁻¹ glucose). Sample throughput for the whole assay of both protein and glucose is 6 h⁻¹. The automated system has been demonstrated for the successive assay of protein and glucose in urine samples taken from diabetic disease patients, with good agreement with the other methods. This developed SI system is an alternative automation for screening for diabetic diagnosis.     

Direct quantification of γ-globulin in human blood serum by resonance light scattering techniques without separation of human serum albumin

We describe the preparation and characterization of functionalized nano-PbS. The functionalized nanoparticles are water-soluble. Reaction of mercaptoacetic acid functionalized nano-PbS with γ-globulin (γ-IgG) results in an enhanced resonance light scattering (RLS) at 385 nm. Based on this, a new direct quantitative method for γ-globulin in blood serum samples without separation of human serum albumin is established. Under optimal conditions, the enhanced RLS intensity is in proportion to the γ-IgG concentration in the range 10-500 ng ml⁻¹. The 3σ limit of detection is 2.75 ng ml⁻¹. The contents of γ-IgG in blood serum samples were determined with a recovery of 97-104% and R.S.D. of 1.5-2.1% (n=6). This method proved to be very sensitive, rapid, simple and tolerant of most interfering substances.     

Measurement of bisphenol A in human serum by gas chromatography/mass spectrometry

The purpose of this study is to establish an easy and accurate method for the determination of bisphenol A (BPA) in the human serum. The samples were applied to the C₁₈ solid phase extraction (SPE) column for clean up of samples. The BPA is conjugated with tetrabutylammonium hydrogen sulfate as the counter ion in alkali solution. The ion paired BPA is moves from the aqueous phase to the organic phase as an ion paired extraction. BPA extracted from human serum were derivatized with pentafluorobenzyl bromide (PFBBr). The derivative was analyzed by gas chromatography (GC)/mass spectrometry (MS) using negative chemical ionization (NCI). The instrumental detection limit of BPA was 5 pg/ml (10 fg). The instrumental response between 0.01 and 100 pg/ml of BPA standards was linear (r²=0.998). The recovery of BPA spiked into human serum was 101.0±4.63 (1 pg/ml) and 100.9±3.75 (10 pg/ml), respectively. The concentration of BPA in the human serum from 20 individuals was 0.54 pg/ml.     

In situ monitoring of gold-surface adsorption and acidic denaturation of human serum albumin by an isolation-capacitance-adopted electrochemical quartz crystal impedance system

Combined measurements of piezoelectric quartz crystal impedance (PQCI) and electrochemical impedance spectrum (EIS) using a suitable isolation capacitance is reported for the first time to monitor in situ adsorption and acidic denaturation of human serum albumin (HSA) on gold electrodes in Britton-Robinson (B-R) buffers. This method provides simultaneously mutual-interference-free and accurate parameters of EIS and PQCI. Effects of surface thiol-modification, electrode-potential and solution pH on HSA adsorption were examined and discussed. Comparative experiments of HSA adsorption in a B-R buffer of pH 6.42 on bare, cysteine- and 1-dodecanethiol-modified gold electrodes revealed that HSA adsorption is more significant on a hydrophobic (1-dodecanethiol-modified) surface. Insignificant electrode-potential effect implied minor electrostatic effects on HSA adsorption. The adsorption amount of HSA at pH 3.28 was found to be notably greater than those at pH 4.84 and 6.42. To characterize HSA adsorption, electrode standard rate constants (k_s) of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple were measured before and after HSA adsorption. The k_s-pH curves on an HSA-modified Au electrode revealed that k_s increased abruptly with the decrease of solution pH below pH ∼4. Moreover, pH-dependent responses of the resonant frequency, the motional resistance, the double-layer capacitance, the capacitance of adsorbed HSA layer and the peak absorbance of HSA solutions at 278 nm all exhibited an inflexion change at pH ∼4, and these findings have been explained on the basis of acidic denaturation of HSA and electrical charges carried by HSA molecules.     

Determination of proteins with Alizarin Red S by Rayleigh light scattering technique

A new protein determination method by enhanced Rayleigh light scattering (RLS) technique has been developed. In acid condition (pH=3.60), RLS of 1,2-dihydroxyanthraquinone-3-sulfonate (Alizarin Red S) can be greatly enhanced by addition of proteins, resulting in two characteristic peaks, 360 and 505 nm, respectively. The new protein assay is based on the RLS enhancement and spectrum change. The optimum condition for the reaction was investigated. The linear range is 0.20-24.9 μg ml⁻¹ for BSA and 0.20-15.5 μg ml⁻¹ for HSA. The detection limits (S/N=3) are 9.59 ng ml⁻¹ for BSA and 9.51 ng ml⁻¹ for HSA. The results of determination for human serum samples were comparable to those obtained by Bradford method. The binding stoichiometry was determined.     

Study on the Langmuir aggregation of fluorinated surfactants on protein

The microphase adsorption-spectral correction (MPASC) technique was described and applied to the study of the interactions of fluorinated surfactants such as potassium perfluorooctanesulfonate (PFOS) and potassium perfluorobutanesulfonate (PFBS) with human serum albumin (HSA). Sodium octanesulfonate (SOS) was also studied as non-fluorinated surfactant. The aggregation of PFOS, PFBS and SOS obeys the Langmuir monolayer adsorption. The results show that the adsorption ratios of surfactants to HSA are PFOS:HSA = 120:1, PFBS:HSA = 205:1 and SOS:HSA = 18:1. The adsorption constants are K_PFOS-HSA = 5.01 × 10³, K_PFBS-HSA = 9.79 × 10² and K_SOS-HSA = 4.03 × 10³. The detection limits are 2.7 mg/L for BSA using PFOS, 3.1 mg/L using PFBS and 3.1 mg/L using SOS. It was found that fluorinated surfactant exhibited stronger interaction with protein than hydrogenated one, and fluorinated surfactant with long hydrophobic chain exhibited stronger interaction with protein than that with short hydrophobic chain.     

A novel lophine-based fluorescence probe and its binding to human serum albumin

The binding of a lophine-based fluorescence probe, 4-[4-(4-dimethylaminophenyl)-5-phenyl-1H-imidazol-2-yl]benzoic acid methyl ester (DAPIM) with human serum albumin (HSA) was investigated by fluorescence spectroscopy under physiological conditions. While DAPIM shows extreme low fluorescence in aqueous solution, DAPIM binding with HSA emits strong fluorescence at 510 nm. The binding constant and binding number determined by Scatchard plot was 3.65 × 10⁶ M⁻¹ and 1.07, respectively. Competitive binding between DAPIM and other ligands such as warfarin, valproic acid, diazepam and oleic acid, were also studied fluorometrically. The results indicated that the primary binding site of DAPIM to HSA is site II at subdomain IIIA. DAPIM can be a useful fluorescence probe for the characterization of drug-binding sites. In addition to the interaction study, because the fluorescence intensity of DAPIM increased in proportion to HSA concentration, its potential in HSA assay for serum sample was also evaluated.     

Detection of Newcastle disease virus with quantum dots-resonance light scattering system

A sensitive QDs-based RLS assay method for the detection of Newcastle disease virus (NDV) antibody has been developed. CdTe quantum dots (QDs) were conjugated with Newcastle disease virus and used as RLS-based probes to detect NDV antibody. The electrostatic interaction between CdTe QDs and NDV resulted in enhanced resonance light scattering (RLS) signal characterized at 555 nm. Upon the addition of NDV antibody, QDs-NDV formed dispersive immunocomplex that can decrease the RLS signal. The decreased RLS intensity at 555 nm (ΔI_RLS) was linearly proportional to the concentration of NDV antibody (C_anti-NDV) in the range of 0.5-50 ng/mL, with correlation coefficient of 0.974 and detection limit of 0.1 ng/mL under the optimization conditions. The proposed method was applied to the determination of NDV antibody in spiked samples with satisfactory results.     

Study of the binding equilibrium between Zn(II) and HSA by capillary electrophoresis-inductively coupled plasma optical emission spectrometry

A new method has been developed for following the interaction between zinc ion and human serum albumin (HSA) by capillary electrophoresis-inductively coupled plasma optical emission spectrometry. Under optimized experimental conditions, the detection limit (3σ) for free Zn²⁺ ion was found to be 1.34 μM by running 11 replicates of the reagent blank. The RSD was less than 3% and the recovery was more than 98.13%. The linear range of zinc ion concentration was between 5.1 μM and 0.3 M. The measured Zn(II)-HSA combination values of n₁ and K₁ for primary binding of Zn²⁺ to HSA were 1.09 and 2.29 × 10⁵ L mol⁻¹, respectively. The measured values of n₂ and K₂ for the non-specific binding of Zn²⁺ to HSA were 8.96 and 6.65 × 10³ L mol⁻¹, respectively. This new method allows rapid analysis of a small amount of sample, simple operation, while avoiding long periods of dialysis and eliminating interference from other metal ions. This method provides a reliable and convenient new way for studying interactions between metal ions and biomolecules.     

Effect of acetonitrile on the hydration of human serum albumin films: a calorimetric and spectroscopic study

A new experimental approach based on the combination of calorimetric and FTIR spectroscopic measurements was proposed to study simultaneously the sorption of water and organic solvent, and corresponding changes in the structure of protein films in the water activity range from 0 to 1.0. Enthalpy changes (ΔH_tot) on the interaction of water with the dried human serum albumin (HSA) in the presence and absence of acetonitrile (AN) have been measured using a Setaram BT-2.15 calorimeter at 298 K. Spectroscopic data on water and organic solvent vapor sorption by the HSA films and the corresponding changes in the protein secondary structure were determined by means of a Bruker Vector-22 FTIR spectrometer. By using a water activity-based comparison we characterised the effect of acetonitrile on the hydration and structure of the HSA films. Acetonitrile (AN) sorption isotherm resembles a smooth curve. HSA film binds about 250 mol AN/mol protein at the lowest water activities. As the water activity increases from 0 to 0.8, the sorption of AN gradually decreases from 250 to 150 mol AN/mol HSA. At a_w > 0.8, the sorption of AN sharply decreases to zero. Acetonitrile decreases markedly the water content at a given a_w. This behavior suggests that the suppression in the uptake of water is due to a competition for water-binding sites on the HSA films by acetonitrile. Changes in the secondary structure of HSA were determined from infrared spectra by analyzing the structure of amide I band. Acetonitrile increases the intensity of the 1654 cm⁻¹ band that was assigned to the α-helix structure. Changes in the intensity of the 1654 cm⁻¹ band agree well with the decrease in water uptake in the presence of AN. An explanation of the acetonitrile effect on the hydration and structure of the HSA films was provided on the basis of hypothesis on water-assisted disruption of polar contacts in the initially dried protein.     

Determination of S-nitrosoglutathione and other nitrosothiols by p-hydroxymercurybenzoate derivatization and reverse phase chromatography coupled with chemical vapor generation atomic fluorescence detection

S-Nitrosoglutathione (GSNO) reacts with the organic mercurial probe, p-hydroxymercury benzoate (PHMB, HO-Hg-(C₆H₄)-COO⁻Na⁺) giving the complex GS-Hg(C₆H₄)COOH (GS-PHMB). This reaction has been studied by UV measurements at 334 nm also in the presence of ascorbic acid and the product of reaction, the GS-PHMB complex, characterized by Electrospray Ionization Mass Spectrometry (ESI-MS) and by Reversed Phase Chromatography (RPLC) coupled on-line and sequentially with a UV-visible diode array detector (DAD) followed by a cold vapor generation atomic fluorescence spectrometer (CVGAFS). The simultaneous presence of PHMB and ascorbate produced a synergistic effect on GSNO decomposition rate that can be observed only above a given concentration threshold of ascorbate (ascorbate/GSNO molar ratio ≥ 180). The results indicated that the formation of GS-PHMB, both in the presence and the absence of ascorbic acid, does not involve the formation of free thiolic species but it takes place through a more complex mechanism. The PHMB derivatives of GSH and GSNO obtained by the present method were found to be identical by ESI-MS. GSSG did not interfere because it was not reduced and derivatized to GS-PHMB. Once complexed by the alkylating agent N-ethylmaleimide (NEM), GSH did not interfere with the derivatization reaction. This ensured a good selectivity of the developed PHMB derivatization system for RSNO determination. Thus, we have optimized the operating conditions for the selective reaction of PHMB with GSNO and other nitrosothiols (RSNOs) in order to determinate RSNOs in human plasma. LODc for RSNOs in plasma ultrafiltrate was 30 nM (injected concentration, 50 μL loop), the DLR ranged between 0.08 and 50 μM and the CV% was 6.5% at 300 nM concentration level. Reduced and oxidized thiols spiked to plasma did not interfere with the measurement of RSNOs. We found that the sampling procedure was critical for the recovery of endogenous and spiked RSNOs. The ultrafiltrate samples of plasma of 8 healthy humans contained 1460 ± 310 CysNO, 1000 ± 330 nM HCysNO and 320 ± 60 nM GSNO if blood was sampled in a mixture NEM/ethylendiaminotetracetic acid (EDTA)/serine-borate complex (SBC), where serine-borate complex is a potent inhibitor of γ-glutamyltransferase, an enzyme involved in the conversion of GSNO into CysGlyNO. In the absence of SBC during the sampling of blood GSNO concentration found in the ultrafiltrate was lower (at level of the determination limit in plasma ultrafiltrate, i.e. 75 nM) and the peak of CysGlyNO appeared, which corresponded to a concentration of 200 ± 60 nM (N = 4 blood samples).     

Pharmacokinetic detection of penicillin excreted in urine using a totally internally reflected resonance light scattering technique with cetyltrimethylammonium bromide

A quantitative analysis method for penicillins including ampicillin (AmP), benzyl penicillin (BP), oxacillin (OA) and amoxycillin (AmO) is proposed that makes use of the totally internally reflected resonance light scattering (TIR-RLS) signal from the penicillin at the H₂O/CCl₄ interface in the presence of cetyltrimethylammonium bromide (CTMAB), and enables the pharmacokinetics of penicillin taken orally and excreted through urine to be monitored. Penicillin is coadsorbed with CTMAB at the H₂O/CCl₄ interface in neutral solution, resulting in the formation of ion associates that display greatly enhanced TIR-RLS signals (maximum at 368–372 nm). This enhanced TIR-RLS intensity was found to be proportional to the penicillin concentration over the range 0.2×10^–6 to 2.2×10^–6 mol L^–1, with limits of determination (3) of 5.0×10^–8 to 7.0×10^–8 mol L^–1. Pharmacokinetics studies performed using the present method show that the excretion of orally-taken ampicillin through urine has a half-time of 1.05 h and an excremental quantum over 8 h of 49.3%, respectively.     

High performance capillary electrophoresis method for determination of ibuprofen enantiomers in human serum and urine

A direct and stereospecific capillary zone electrophoresis (CZE) method for quantification ibuprofen enantiomers in biological matrices: human serum and urine, has been developed. Chiral separation of the enantiomers of ibuprofen and (+)-S-indobufen [(+)-S-INDB, internal standard, IS] was obtained in an uncoated silica capillary filled with a background electrolyte (BGE), consisted of heptakis 2,3,6-tri-O-methyl-β-cyclodextrin (TM-β-CD) in buffer of pH 5.0. The complete enantioselective analysis of ibuprofen and its 1-hydroxy metabolite confirmed appropriate specificity of the method. The electrophoretic parameters: electroosmotic (μ_EOF) and electrophoretic (μ_ep) mobility and resolution factor (R_s) were determined. Extraction procedures with organic solvent and solid phase extraction (SPE) with C₁₈ stationary phase for isolation of enantiomers from biological fluids were compared. SPE method for further studies was chosen. Stereoselective extraction of IBP enantiomers from serum at basic pH has been discovered. Validation of the method was carried out. Calibration curves of ibuprofen enantiomers were linear in the range of 0.1-25.0 μg/ml in serum and of 0.5-250.0 μg/ml in urine. Recovery of both enantiomers from serum and urine amounted 74-86 and 90-98%, respectively. Intra- and inter-day measurement precision and accuracy were below 15%. Limits of detection for IBP enantiomers amounted 0.05 and 0.25 μg/ml in samples of serum and urine, respectively. Limit of quantitation was also estimated. IBP enantiomers proved to be stable following three freeze and thaw cycles and during storage in autosampler at ambient temperature. The validated methods enable pharmacokinetic studies of enantiomers in both media. The elaborated HPCE method can be alternative to HPLC.     

Synchronous fluorescence determination of protein with functionalized CdS nanoparticles as a fluorescence probe

Nanometer-sized fluorescent particles have been successfully synthesized. A synchronous fluorescence method, with high sensitivity and selectivity, has been developed for rapid determination of protein with functionalized CdS as a fluorescence probe. When Δλ=260 nm, maximum synchronous fluorescence is produced at 274 nm at pH 7.0. Under optimal conditions, the calibration graphs are linear over the range 0.1-3.0 μg ml⁻¹ for bovine serum albumin (BSA), 0.1-11.0 μg ml⁻¹ for γ-globulin (γ-G) and 0.1-1.4 μg ml⁻¹ for human serum albumin (HSA), respectively. Limits of determination were 0.01 μg ml⁻¹ for BSA, 0.019 μg ml⁻¹ for γ-G and 0.021 μg ml⁻¹ for HSA, respectively. The relative standard deviations of seven replicate measurements were 1.8% for 1.0 μg ml⁻¹ BSA, 2.2% for 1.0 μg ml⁻¹ γ-G and 2.3% for 1.0 μg ml⁻¹ HSA.