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Understanding the Role of Synthesis Conditions on Gold and Silver Nanoparticles Properties

Synthesis of Gold Nanoparticles

Understanding the role of synthesis reaction conditions on the properties of Gold and Silver Nanoparticles

In this practical you will synthesise gold and silver nanoparticles using conventional inorganic reductants and biomolecules as reductants. The success of your syntheses will be measured by the spectroscopic data that you collect.  

NOTE: Not all details for the procedures are given- before the laboratory you should find research papers dealing with the same/ similar syntheses and see what ‘tricks’ are used to get the experiments to work. You will also need to find reference articles to find out how to calculate the sizes of the particles measured from the spectroscopic data collected. 

A careful approach to the laboratory work will generate good samples for analysis. The aim of the practical and associated report is to:
(a)Give you an appreciation of the size and behaviour of metallic nanoparticles;
(b)Give you an understanding of the care and control required to prepare such particles;
(c)Give you an understanding of the mechanism behind colour production for such small particles, and Help you to understand the nature of biomolecule-nanoparticle interactions in the generation and stabilisation of such particles.

Synthesis of Gold Nanoparticles:

Route 1: using a conventional reductant

It is advisable to start with smaller volumes of solution to work out appropriate conditions. Take care to measure ACCURATELY. 
Make up a solution of 0.01M NaBH4 (you will need this for several of the experiments so ensure that you have allowed for this when you prepare the solution). Calculate how much solution you need to add to reduce the gold solution. Try two different approaches to generating gold nanoparticles. Either add the required amount dropwise to 5 mL of 2.5x10-4M aqueous solution of HAuCl4, or add all in one go.  Then mix by shaking gently. Solution color will change immediately from colorless to ruby red. The size of synthesized gold nanoparticles should be 20 ± 2 nm. If a black colour is encountered, reduce the amount of the reducing agent added until a stable red coloured solution is obtained. 

Requirements: small flask, pipettes. HAuCl4, NaBH4

Route 2: using a Cationic surfactant and conventional reductant 0.250 mL of an aqueous 0.01 M solution of HAuCl4.3H2O will be added to 7.5 mL of a 0.10 M CTAB solution in a test tube (glass or plastic) (make sure that the CTAB solution is warm to prevent it crashing out of solution in the next step). The solutions will be gently mixed by inversion. The solution appears bright brown-yellow in colour. Then, 0.600 mL of an aqueous 0.01M ice-cold NaBH4 solution will added all at once, followed by rapid inversion mixing for 2 min. Care should be taken to allow the escape of the evolved gas during mixing. The solution develops a pale brown-yellow color. Then place the test tube in a water bath maintained at 25 °C for 2 h. The size of synthesized gold nanoparticles should be 4 ± 2 nm.  

Route 1: Using a Conventional Reductant

These particles can also be used for the synthesis of gold nanorods (the ‘seed’ solution).

Requirements: test tubes/ small flasks, accurate pipettes and measuring vessels, 0.01M HAuCl4.3H2O, 0.10M CTAB, 0.01M NaBH4

Route 3: Glucose Mediated synthesis of gold nanoparticles Typically, 9 mL of an aqueous solution of 2.5 X 10-4 M HAuCl4 will prepared in a conical flask. 1 mL of 1 M D-glucose will be added to this solution (take care in the making of this solution, glucose should be added incrementally to favour dissolution). The solution will be heated to 60 °C, 20 μL of 1 M NaOH will be added, and the resulting solution will be subsequently stirred rapidly for 10 s. The color of the solution will immediately change to ruby red, indicating the formation of gold nanoparticles. The particles are believed to be stabilized by a monolayer of gluconic acid on the surface and thus are negatively charged. The size of gold nanoparticles should be ca 12 ± 4 nm. 

Requirements: small flasks, accurate pipettes, HAuCl4, 1M D-glucose, 1M NaOH

Route 4: Using an antibiotic to synthesise gold nanoparticles

Add equal volumes (100 microlitres) of the antibiotic provided (1 x 10-2 M) and the 0.01 M HAuCl4 solution to 9.8 ml water. Divide into 2. Cover the samples. Keep one sample at room temperature and the other at 60oC for two hours or more until the colour has stabilised. 

Route 5: Using sodium citrate to synthesise gold nanoparticles (Turkevitch method) The method is well established and varying the concentration of reactants, ratio of reactants and temperature of reaction modifies the size of the nanoparticles formed. Ensure that you use solutions containing gold and sodium citrate that are heated to similar temperatures. Note down the conditions that YOU use and record these in the lab write up. To 9.5ml of gold solution (2.5 x 10-4 M) that has been heated to between 70 and 100 oC (temperature noted), add 0.5ml of sodium citrate (concentration either 0.5 or 1% by weight, your choice) whilst stirring vigorously and observe what happens to the colour of the solution with time. Note all changes.

Requirements: small flasks, magnetic stirrers, small stirrer bars, sodium citrate, 2.5 x 10-4 M gold solution.  

Route 6: Synthesis of Citrate Stabilized gold nanoparticles:


This method is another citrate stabilised synthesis of gold nanoparticles which can be used to create seed particles for Route 8. In a flask mix, HAuCl4 and Citrate to form a 20ml solution where the concentration of HAuCl4 is 2.5x10-4M and Citrate is 1x10-4M. Add 60 μL of freshly prepared ice-cold 0.1M sodium borohydride solution while stirring vigorously. Transfer to a sample tube and wrap in tin foil and leave until a red colour develops.

Route 2: Using a Cationic Surfactant and Conventional Reductant

Requirements: 20ml flasks, accurate pipettes, 0.01 x 10-4 HAuCl4, Sodium Citrate, 0.1M sodium borohydride solution.

Route 7: Synthesis of Gold nanorods: 


This route is very difficult to achieve good results. It will pay to find literature methods to ensure that you have all of the information that you need to perform this part of the practical. Gold nanorods can be synthesized by a three-step seeding protocol. Note all colour changes that occur. Specifically, two 20 mL conical flasks and one 100 mL conical flask (labeled A, B, and C, respectively) are taken. To these flasks will be added 9 mL (in flasks A and B) and 45 mL (in flask C) of growth solution containing a mixture of 2.5 X 10-4 M HAuCl4 and 0.1 M CTAB solutions. To these solutions will be added 50 μL (flasks A and B) and 250 μL (flask C) of 0.1 M freshly prepared ascorbic acid, and the resulting solutions will be stirred gently. Now add 1.0 mL of the seed solution (see route 2 above) to sample A (step 1) and mix. After 15 s, mix 1.0 mL of sample A with sample B (step 2). 

Wait 30 seconds and then add a 5.0 mL portion of sample B to sample C after 30 s (step 3).  In all cases, each flask should be gently stirred to homogenize the solutions. The solution in flask C should be kept at 25 °C for a period of 16 h before measuring the particle size.

Note- if you wish to use an alternative approach, perhaps using silver ions as a dopant then share with me (a week before) the reaction protocol and I can check materials will be available for you when we come to the labs. 

Requirements: 2 x 20ml flasks, 1 x 100ml flask, accurate pipettes, 2.50 x 10-4 HAuCl4, 0.10M CTAB, 0.10M ascorbic acid This route generates another different morphology of gold particles. To get good results you will want to check the literature to ensure you know all of the information that you will need to perform this practical. Gold nanostars are synthesized by a one-step seeding protocol. Note any colour changes that occur.

Take a 20mL conical flask, to this flask add the growth solution which contains a mixture of 0.2 ml 1 X 10-2 M HAuCl4 and 4.7 ml 0.1 M CTAB solutions. To this solution 30 μL of 0.01M silver nitrate is added and stirred gently. This is followed by 32 μL of  0.1M freshly prepared ascorbic acid and the resultant solution stirred gently. 

Route 3: Glucose Mediated Synthesis of Gold Nanoparticles

Now add 50 μL of the seed solution (see route 6 above) to this flask and mix.

Requirements: 1 x 20ml flasks, accurate pipettes, 2.50 x 10-4 HAuCl4, 0.10M CTAB, 0.10M ascorbic acid, 0.01M Silver Nitrate Synthesis of silver nanoparticles:

Route 1: using a conventional reductant:

Place 10 mL of 10-3M aqueous silver sulfate solution and 10 mL of 10-3M/ 10-4M NaBH4 in a conical flask and dilute to 100 mL with deionized water. A colour change to pale yellow should occur. If this does not occur at room temperature, warm gently to get the reaction to occur. 

Requirements: 100mL volumetric flask, accurate measuring cylinders/ pipettes, 1 x 10-3 silver sulfate, 0.001M NaBH4, 0.10M KOH

Route 2: biomimetic approach using tyrosine:

Place 10 mL of 10-3M aqueous silver sulfate solution and 10 mL of 10-3M aqueous tyrosine in a conical flask and dilute to 100 mL with deionized water. Add 1mL of 10-1 M KOH solution and boil the solution until the solution changes colour, which will indicate the synthesis of silver nanoparticles. 

Requirements: 100mL volumetric flask, accurate measuring cylinders/ pipettes, 1 x 10-3 silver sulfate, 1 x 10-3 tyrosine, 0.10M KOH

Phase transfer Synthesis of gold nanoparticles:

Synthesize gold nanoparticles using any of the methods described above. In principle, the routes to NP using sodium borohydride should work best (why!?). Place 5 mL of a gold nanoparticle containing suspension in a small vessel. Add 5 mL of a 10-2 M solution of octadecylamine dissolved in chloroform to give immiscible layers of the red colored gold hydrosol on top of the colorless organic solvent. Shake to see if you get transfer between the aqueous and non-aqueous phases as evidenced by red coloration for gold nanoparticles or yellow/brown coloration for silver nanoparticles of the organic phase. 

Requirements: small pots/ red top tubes, centrifuge tubes, separating funnel, accurate pipettes, measuring cylinders, nanoparticles as synthesized before, 10-2 M octadecylamine dissolved in chloroform, or equivalent organic solvent. All particles made will be assessed by UV/ visible/ NIR spectroscopy using spectrometers in the Rosalind Franklin Building/ ISTEC and must be done within the lab sessions. For all gold syntheses- apart from the nanorod synthesis record data between 400-700nm. For the nanorods- record data between 400-1100nm and for the silver nanoparticles record data between 300-600 nm. 

Report Questions: 

Note all work will be run through TURNITIN to check for plagiarism. In particular, questions 3 and 4 require a lot of effort and significant amounts of literature work, data analysis/ writing up. You MUST find at least one research paper that describes how to convert the spectroscopic absorbance data into sizes. First, make a table of your data and add literature data for the results of comparative syntheses found in the literature.  Then make a figure (for each metal) that includes all of your processed spectra (for all experiments conducted) and include as an addendum at the end of your report. 

1.What is a nanoparticle? When do the properties of a nanoparticle such as gold or silver become those of the bulk metal? Consider a minimum of TWO different properties aside from colour generation that is discussed in detail below. 

2.Make a table of your results (peak maximum, size etc.) and add literature data for the results of comparative syntheses found in the literature. Remember to include references for the data taken from the literature.  

3.In the synthesis of nanoparticles of metals, what parameters affect growth? Illustrate your answer with your own data and that found in the primary literature. Make figures to illustrate your arguments. Your answer should include at least one figure and one table of data.  

4.Explain the theory behind colour generation in Au and Ag nanoparticles. Use your own data show whether the colour observed is affected by particle size and/or capping agents. Use figures and/or tables to illustrate the points you make.

5.By comparison with data in the primary literature indicate how successful your syntheses were (use two or three synthesis routes for which you have data). Provide explanations for your responses. 

6.What role(s) do CTAB, glucose, citric acid, tyrosine and cefaclor play in the formation and stabilization of your nanoparticles? Describe the structures of these molecules and indicate which parts of the molecule are effective and why. 

Explain the chemistry behind all the colour changes that occur during the synthesis of gold nanorods using route 7. 

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