Borohydride is very strong reducing agent. Use dilute conct of hydrazine hydrate

What is the color of your gold sol? Please post a picture. Is this a single or double beam spectrometer?

The primary indication about the formation of nanoparticles is colour. If the colour of suspension is pinkish (light ruby), then it was first clue about the formation of nanoparticles. Please check the suspension colour first. After that, recorded the uv-vis spectra of synthesized nanoparticles suspension.

Spectra shows the absence of any plasmon resonance peak in the spectra. Now you have to check whether sufficient nano particles are prepared. The change in suspension color will show the formation and density of nanoparticles.

@ Jagdeep Singh

A 2 nm gold sol will be yellowish in color not pinkish or purple.

Thank You all for your response

@ Alan F Rawle. Please find attached the picture of gold nanoparticle solution in water. I have taken at two different concentrations: 3mg/ml and 1 mg/ml. We use a Biotek Cytation-3 96 well plate reader. I guess it is a single beam spectrometer.

Please let me know if the nanoparticles are ok ?

If they are ok, Is the UV-vis spectrum acceptable ? If not, what would you recommend ? All suggestions are really appreciated

Thank you very much

The brownish color suggests aggregation and large particles. BTW, I don't personally like borohydride reduction as this leads to inevitable contamination of the system by boron which cannot be removed. Easily shown by ESCA/XPS. I used (5%) hydrazine hydrate reduction when I made gold colloids in the 70's (that's the 1970's not my 70's which are to come, hopefully). Only products are N2 and water. I suggest a DLS experiment to confirm the size distribution. Stabilization is key here - you need to look at both charge ('electrostatic') and steric stabilization. I favor the latter with small gold sols - a 50 kDa PEG or PEI in the optimum concentration is my favored steric stabilizer.

Small final sizes are favored by dilute concentrations of precursor and reductant as well as an appropriate 'chain stopper' (the steric stabilizer acts like this). pH control can aid but chloroauric acid (the usual Au precursor) is highly acidic. Low concentrations and small sizes make the surface plasmon resonance much harder to see though.

Thanks Alan for your response.

My particles show a Zeta of -40 mV when dispersed in water. That suggests the possibility of electrostatic stabilisation. Also I add a thiol ligand to the gold salt even before I add NaBH4. Thiol ligand may act as stabiliser too. DLS size distribution, I have to do.

Can I ask you what does the absence of any shoulder in the UV-Vis spectrum suggest about the quality of the particles ?

Thank You

A zeta potential of - 40 mV is completely inadequate to keep 2 nm particles stable. You need an order of magnitude closer to 150 mV. See other discussions on ResearchGate. For example:

https://www.researchgate.net/post/How_do_I_interpret_zeta_potential_values

Absence of shoulder tells you no nanoparticles or they're extremely dilute. The giveaway is the color though. At 2 nm you expect a faint straw color. Tell me the outcome of your DLS results although that will only tell you about the material in suspension. Carry out a Stokes' Law calculation for Au so you'll know what can remain in suspension.

Borohydride is very strong reducing agent. Use dilute conct of hydrazine hydrate