I am using the following method for the synthesis of gold nanorods, but not able to get the expected result.
The growth solution was prepared at 25−30 °C. HAuCl4 (5.0 mL, 1.0 mM) was added to 5.0 mL of cetyltrimethylammonium bromide (CTAB; 0.2 M). Following this, AgNO3 (250 μL, 4.0 mM) was added, and the solution was gently shaken. HCl (8.0 μL, 37%) was introduced to obtain a pH of 1−1.15.Then, 70 μL of ascorbic acid (78.8 mM) was added to the solution with gentle shaking until the solution was clear. Immediately afterward, ice-cold NaBH4 (15 μL, 0.01 M) was injected to the unstirred growth solution and allowed to react for 6 h. The resulting gold nanorods had dimensions of 18.0(±2.0) × 4.5(±0.5) nm, with an aspect ratio of ∼4, and longitudinal plasmon absorption at ∼770 nm.
Can any suggest me where i might be wrong....
or anyother alternative method to get the nanorods of the specific size...
Thank you..
You need seed mediated method:
Firstly synthesize gold seeds (CTAB+HAuCl4+NaBH4), then add the seeds into your growth solution, which is the composition mentioned by you. However, you don't add NaBH4 in the growth solution as this immediately will reduce all gold to spheres! NaBH4 is a too strong reducing agent, that's why you added the ascorbic acid to reduce Au(III) to Au(I) (changing the color from yellow to colorless). This will induce slow growth on the seeds to form the AuNR. Don't stir the solution after adding the seeds to get well shaped AuNR (slight shaking for 10s to mix the seeds, than put aside in water bath).
There is heaps of literature actually on gold nanorods... most people will cite the Murphy group, as they have investigated AuNR growth and protocols intensively.
This is their initial paper on reliable AuNR growth of high AuNR yield:
http://pubs.acs.org/doi/pdf/10.1021/la049463z
where do you get that synthetic method??
I don´t think that the seed mediated method it´s the best option in this case because the minor axis of the obtained nanorods is generally around 10nm.
http://pubs.acs.org/doi/abs/10.1021/cm020732l
Just Follow this paper,.. I think no need of HCl and the addition of NaBH4 in the growth solution. As already said by Andreas Ott, you just follow seed-mediated process.. Synthesize seed particles using CTAB as the capping ligand.. Use the freshly prepared seeds for the growth of nanorods. If you use the aged seed solution, that will also affect the growth of gold nanorods..
I would like to know why you introduce HCl. after you acidify the solution to pH 1-1.15, does the ascorbic acid reduced the Au3+ ions to Au+ ions? that is does the addition of ascorbic acid turns the colour of the solution changed from yellow to colourless ? I am unsure about the reduction ability of ascorbic acid in acidic solution because Ascorbic acid has pKa values of 4.2 and 11.6. At more acidic pH it may not reduce Au3+ ions to Au+ ions and hence u may not observe the growth of gold nanorods.
well I have tried with the seed mediated method as well, but again i am unable to get the nanorods of (18 * 4.5nm).
Rajat i am expecting the nanorods of specific aspect ratio (18* 4.5 nm), with two absorption peak (small between 400-600nm and broad peak between 700-900nm).
Well Amal Raj i add the HCl to acidify the solution. bez the pH of the solution before addition of HCl is more than 4 so to adjust the pH i added the HCl. bez as per procedure the pH of the solution should be around 1-1.5.
Andreas, can i get the procedure in much more detail, as i am unable to open the link you mentioned.
Thank you all..
Have you used freshly synthesized seed particles in the seed mediated process?.. I too synthesized the AuNRs via seed mediated process but in ph 4.5 not in acidic media.. I dont know why u want to keep the pH of AuNRs as 1 - 1.5... You can vary the pH and try it.. pH may be the problem. Good luck
The pH indeed doesn't need to be lowered - I have tried both. HCl is needed for higher anisotropie. Such thin NR (5 nm diameter) are not easily made...
The basic procedure is (similar to all NR receipts):
seeds: 7.5 ml 0.1M CTAB+ 0.25 ml 10mM HAuCl4 + 0.5 ml 10mM NaBH4 --> 1.5h aging @ 26°C
Then growth solution: 14.5ml 0.1 M CTAB + 0.6ml 10mM HAuCl4 + 0.09ml 10mM AgNO3 + 0.096ml 0.1M Ascorbic Acid + 0.15ml of seeds, gentle shaking after each addition, let stand still for over night
This will give you 30x9 nm AuNR with approx. 30% of reacted gold content. For smaller AuNR you can try to increase seed concentration or play with HCl, or concentrations of the growth solution.
The literature might give you more hints.
Thank you Amal and Andreas, for the suggestion.
I guess the NaBH4 should be prepared fresh and in ice cold solution.
If you don't mind that some copper is in your Gold rod,, you could try this variation http://pubs.acs.org/doi/pdf/10.1021/jp810433e, too.
For me HCl worked always for making the rod "fatter". I obtained sizes around 30*90nm. At some point I tried HNO3, which makes the rods really thin ~5nm, but it was somehow unreliable.
Good luck!
Thank you Andreas,
I just wanted to know the reason of adding copper to the gold rod.
It was for generating alloyed particles. But it made the rods thinner for some reason...
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