Nanokinetic STUDIES On SYNTHESIS of Gold Particles by Classical Citrate REDUCTION METHOD

NANOKINETIC STUDIES ON SYNTHESIS OF GOLD PARTICLES

BY CLASSICAL CITRATE REDUCTION METHOD

Anirban Chakraborty¹, Sampa Chakraborty², Basab Chaudhuri³, Sekhar Bhattacharjee*

Department of Chemical Engineering, University of Calcutta

92, A.P.C. Road, Kolkata-700 009, India

 *Corresponding author, telephone: +91 033 23508386; e-mail: sbcheme@caluniv.ac.in;

Anirban Chakraborty¹: anirbancu@yahoo.co.in, Sampa Chakraborty^2: sampac.2008@gmail.com,

Basab Chaudhuri³: basab_chaudhuri@hotmail.com

ABSTRACT

Although vast information on preparation of metal nanoparticles and their applications in bioimaging, biosensors, nanomedicines and diagnostic products has been reported in recent past, experimental data on the fundamental parameters of reaction kinetics or process engineering of metal nanoparticle synthesis are not available till date.  In this paper we present a novel approach to study the reaction kinetics and the dynamics of surface morphology during synthesis of metal nanoparticles. Synthesis of gold nanoparticles (GnPs) by reduction of chloroauric acid with trisodium citrate (Turkevitch method) was selected as the model reaction where the citrate acted as a reducing agent and stabilizer. The reduction reaction proceeded through rapid color changes due to change in surface plasmon resonance (SPR) of the colloidal gold solution.  Our objectives were to determine the reaction order and kinetic rate constants for the pseudo-homogeneous reduction reaction and to study the effects of process operating conditions on the GnP morphology during the nucleation and maturing stages. Influences of the process parameters- molar ratio of reactants (0.33-5.3), temperature (333-373 K) and  pH (3-7) of the reaction medium on the kinetic parameters  and GnP morphology  were investigated for identification of optimal reaction conditions and suitability of mature GnPs for biomedical applications.

EXPERIMENTAL SECTION

Preparation of GnPs 

Gold nanoparticles were prepared by the well known Turkevitch method [1]. 50 ml 0.254 mM solution of chloroauric acid (pH= 3.18) was heated and stirred continuously on a hot plate magnetic stirrer. 0.5 ml of 0.34mM trisodium citrate (TSC) solution was added dropwise to the boiling chloroauric acid solution. On addition of citrate solution colour of the solution gradually changed from colourless to grey to black to violet to purple to maroon red and eventually to wine red in about 300s.    Aliquot samples of the reaction medium (from colourless to wine red) were collected at definite time intervals using a micropipette and kept in plastic reavoils in freezing condition (275-277K) for further analysis.

RESULT AND DISCUSSION

Kinetics and particle formation study

Concentration of chloroauric acid in the samples collected during reaction was measured directly by monitoring the UV spectroscopic absorbance value at 212.5 nm  and cross-checked by the iodometric titration method [2,3,4].  Baseline experiments were conducted at M= 1.33 ( 50 ml 0.254 mM solution of chloroauric acid and 0.5 ml of 0.34mM sodium citrate) under boiling conditions.  From our repeated study it was observed  that concentration of chloroauric acid  decreased exponentially with time suggesting a pseudo-first order reaction kinetics [C = C₀exp (-kt)] for the reduction reaction (Figure 1) . Wavelengths corresponding to peak absorption (λ _max) vs. time data for the samples collected at different stages of reaction are plotted in Figure 2.  Similar observations have been reported by other investigators [5].

 

Fig 1: Concentration of precursor chloroauric acid vs time profile (M=1.33)

Fig 2: SPR wavelength (λ _max) vs. time  under baseline reaction conditions

 

 

Hydrodynamic mean diameters of particles in the samples were recorded with DLS instrument.  Mean diameters of GnPs in samples collected during reaction are plotted against time in Figure 3. DLS & AFM studies (Figures 4,5, 6 and 7)  show that  gold particles gradually build up during the course of  reaction. We could not detect any particle in the samples collected, either by DLS or AFM, within the first 30s of reaction.  Very first GnPs appeared in the reaction medium about 30 s after addition of trisodium citrate. These particles are irregular, non-spherical particles of about 24.5 ± 2.9 nm mean hydrodynamic diameter (Figures 3 and 4). These particles formed the nuclei for further growth of matured gold nanoparticles. With time these non-spherical nuclei became larger and more spherical in shape (Figures 5 and 6) . After about 300 s particles were matured to nearly perfect spherical shape with about 46.31 ± 5.2 nm diameter (Figure 7). Similar observations were also reported by Kemling et al [6,7].

Fig 3: Particle diameter vs time plot during reaction

Fig 4: AFM image of GnPs                                          Fig 5: AFM image of GnPs

 20 s after TSC addition (Gray solution)                                  60 s after TSC addition (Dark solution)

Fig 6: AFM image of GnPs                                                 Fig 7: AFM image of GnPs

 150 s after TSC addition (Violet solution)                       300 s after TSC addition (Wine red solution)                             

 

Change of initial concentration of reactants

Molar ratio of reactants in the baseline experiment was 1.33 ± 0.053 (0.254 mM solution of chloroauric acid and 0.34 ± 0.02 mM sodium citrate solution).  Effects of change in the initial molar ratio of the reactants on peak wavelength(λ_max) and  particle diameter of GnP in the final solutions (300s after addition of TSC) are shown in Figures 8 and 9 respectively.

Fig 8 : Variation of λ _max with  initial molar ratio of the reactants (M):  Curve A: HAuCl­₄ concentration =1.016mM to 0.03175 mM and fixed TSC concentration=0.34 mM.  Curve B: Fixed HAuCl­₄ concentration =0.254mM and TSC concentration= 0.08-1.36 mM

 

Fig 9: Variation of  mean particle diameter  with  initial molar ratio of the reactants (M):  Curve A: HAuCl­₄ concentration =1.016mM to 0.03175 mM and fixed TSC concentration=0.34 mM.  Curve B: Fixed HAuCl­₄ concentration =0.254mM and TSC concentration= 0.08-1.36 mM

In Figures 8 and 9, as  the two curves , A and B, almost overlap on each other it may be concluded that in the concentration range studied in our experiments, wavelength for peak absorption of final nanogold colloidal solution, which is indicative of the shape, size and aspect ratio of the GnPs formed, depends only on the initial molar ratio of the reactants and is independent of the molar concentrations of either chloroauric acid or trisodium citrate. Investigations by the other researchers [5,7] on mean particle diameter against molar ratio also substantiate our experimental findings and conclusions.  

 

 

Fig 10: A plot of k vs molar ratio (M).  Curve A: HAuCl­₄ concentration =1.016mM to 0.03175 mM and fixed TSC concentration=0.34 mM.  Curve B: Fixed HAuCl­₄ concentration =0.254mM and TSC concentration= 0.08-1.36 mM

The pseudo-first order rate constant (k) values for different molar ratios of the two reactants are plotted in Figure 10.  Change in molar ratio has a significant impact on the reaction kinetics. Maximum value of rate constant of the reduction reaction was observed for the baseline molar ratio M = 1.33 ± 0.05.  If  the molar ratio is increased or decreased by changing  either the concentration of chloroauric acid or  the concentration of trisodium citrate, values of pseudo-first order rate constant (k) always decreased from the maximum value (0.0075 s^-1) indicating that the  reduction reaction is fastest under  the baseline process conditions. 

Change of temperature of the reaction medium

Experiments were conducted at baseline molar ratio of 1.33 ± 0.053  while the temperature of the reaction medium was varied between 373K and 343K.  In all these cases exponential decrease in concentration of chloroauric acid with time was observed (Figure 11)  and the pseudo-first order rate constants were calculated. From an Arrhenius plot of  the computed rate constants and temperature, activation energy for the  liquid phase homogeneous reaction leading to formation  of gold nanoparticles was estimated to be 51.64 kJ/mol.

Fig 11: Time- concentration profile of HAuCL₄ during  for different reaction temperatures; M=1.33.

Change of pH of the reaction medium

In the baseline experiments, pH of the precursor 0.254 mM chloroauric acid solution is 3.18 while that of 0.34mM trisodium citrate is 8.44.  During reaction pH of the reaction medium remained nearly constant at 3.6. To investigate the effects of pH of the reaction medium on the reduction kinetics, experiments were conducted under boiling condition at baseline molar ratio of  the reactants (1.33 ± 0.053) while the pH of the reaction medium was adjusted between 3 and 7 by adding required amounts of NaOH or HCl. The time-concentration profiles of  chloroauric acid at different pH are shown in Figure 12.  In the pH range 3.6-5.2, well-defined first order reaction kinetics was observed.  At pH higher than 5.2 , the time-concentration profile showed  distinct deviation from the first order kinetics suggesting a shift in the reaction mechanism for the reduction reaction[5,8,9].   

Fig 12: Time- concentration profile of HAuCL₄ for different pH of the reaction medium; M=1.33

CONCLUSION

Aliquot samples collected from intermediate stages of reaction were studied with UV-vis Spectroscopy, Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM). Time-variant precursor concentrations in the reaction medium were measured directly by recording absorbance of the reaction medium at 212.5 nm and cross-checked by iodometric estimation method. During reaction, chloroauric acid concentration in the reaction medium decreased exponentially with time suggesting an apparent pseudo-first order reaction kinetics (k=0.0075 ± 0.016 s^-1) while the SPR peak of GnPs in the reaction medium decreased from 556.5 nm to 520 nm. During the nucleation stage (< 35 s) non-spherical particles with mean hydrodynamic diameter of 24.5 nm were detected which formed the nuclei for further growth and maturing to spherical GnPs after about 300 s. Mole ratio of reactants (Citrate: Chloroauric acid) in the baseline experiment was maintained at 1.33 ± 0.053  because, the GnP size and spherical symmetry obtained under this condition are suitable for biological applications. Influences of process parameters like temperature, pH and molar ratio of the reactants on the kinetic rate parameters and morphology of the formation of GnP were investigated.

 

NOMENCLATURE

C-    Concentration of chloroauric acid at time t(mM)

C₀ -  Initial concentration of chloroauric acid (mM)

C_s – Concentration measured by UV- Vis Spectrophotometer(mM)

C_i –  Concentration measured by Iodometry (mM)

k  -   Pseudo-first order reaction rate constant (s^-1 )

M – Initial Molar Ratio (Citrate/ HAuCl₄)

t -    time (s)

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