Dear colleagues,

You are forgetting another important fact, that the position of the Fermi level of metal nanoparticles can be drastically changed in the presence of a nucleophilic reagent (borate or citrate ions – using as a reductants) that donates an electron pair to the unoccupied orbitals, which exist on the nanoparticles surface. Therefore, the blue shift of the surface plasmon peak (absorption maximum) of Au nanoparticles, can be explained by the increase of the effective concentration of free conductive electrons in the nanoparticles. On the other hands, change in the electron density in the nanoparticles can influence on the optical properties of colloidal metals. See references:

1. V.V. Vodnik, J.M. Nedeljković, Influence of negative charge on the optical properties of silver sol, J. Serb. Chem. Soc., 65 (2000) 195-200.

2. V.V. Vuković, J.M. Nedeljković, Surface modification of nanometer-scale silver particles by imidazole, Langmuir, 9, (1993) 980-983.

Dr Vesna Vodnik, Vinca Institute of Nuclear Sciences

Based on Mie’s (1908) theory, nanoparticles with different sizes should demonstrate different optical properties due to the difference in the SPR (Surface Plasmon Resonance). Silver Nanoparticle spherical-shaped will exhibit a single SPAB (Surface Plasmon Absorption Band), whereas anisotropic particles will exhibit two or three bands depending upon their shape. Larger particles will in turn exhibit additional bands corresponding to quadrupole and higher multi-pole plasmon excitation. SPAB of metal nanoparticles shifts to longer wavelengths with increasing particle size, as reported by Brause et al. (2002). The SPAB of metallic nanoparticles displays the blue shift with decreases in the particle diameter, as elucidated by Heath (1989).

http://www.researchgate.net/group/Green_Nanotech/

What are the gold nanoparticles themselves? Are you sure that the UV light incidence was exactly at the same spot as in the previous experiment? Are you sure that the nanoparticles were kept clean and no contamination has adsorbed?

One possibility is that there could be possible precipitation of larger size gold nanoparticles (or aggregates) in you solution leading to change in the particle size distribution in your solution (the suspended part). Possibly, after few days the solution consists of higher percentage of smaller size nanoparticles compared to the initially synthesized solution. Dynamic light scattering measurements of your solution can give you a better insight.

Thank you Dr Robin Klupp Taylor for your valuable information

I really like Dr. Robin's comments and totally agree. I bat the sedimentation is probably the case and it is unlikely due to the instrumental error.

I think you want to synthesize large gold nanoparticles, may be near 100 nm. The large gold nanoparticles are not very stable after synthesis capped by citrate (if you use it as a reducer and capper). It's possible that some of the larger particles sedimentate after 20 days, so the UV detect smaller particles with a blue shift. Do you observe any change of the spectrum shape, become wider or narrower?

@swati Jain: My nanoparticles are getting a blue shift. Please go through my question once again. The thing which you are telling is in case of red shift.

@robin K Taylor: Thak you very much for your nice explanation and also about the "UV analysis" this ;). @ shiv: thank you very much:

@all: thank you for your replies

Dear colleagues,

You are forgetting another important fact, that the position of the Fermi level of metal nanoparticles can be drastically changed in the presence of a nucleophilic reagent (borate or citrate ions – using as a reductants) that donates an electron pair to the unoccupied orbitals, which exist on the nanoparticles surface. Therefore, the blue shift of the surface plasmon peak (absorption maximum) of Au nanoparticles, can be explained by the increase of the effective concentration of free conductive electrons in the nanoparticles. On the other hands, change in the electron density in the nanoparticles can influence on the optical properties of colloidal metals. See references:

1. V.V. Vodnik, J.M. Nedeljković, Influence of negative charge on the optical properties of silver sol, J. Serb. Chem. Soc., 65 (2000) 195-200.

2. V.V. Vuković, J.M. Nedeljković, Surface modification of nanometer-scale silver particles by imidazole, Langmuir, 9, (1993) 980-983.

Dr Vesna Vodnik, Vinca Institute of Nuclear Sciences

Robin. You can expect similar effect if you are using nucleophilic reagent to donates an electrons.

Anjul. I was prepared Ag and Au nanoparticles using trisodium citrate dihydrate and sodium borohydride as a reducing agents.

I am nether using citrate nor sodium borohydride but I am just using some green plants for the synthesis. The chemical compositions of the plant I am using is still not reported anywhere to my knowledge and am working on that.

I think I shall go for DLS and compare the results. Like this I can be in a better position to really comment upon. Anyways thanks a lot Vesna

Did you made any TEM images to see changes in shape of Au nanoparticles?

can I ask a question about gold nanoparticles? in our lab I planned to use 40 nm gold nano in citrate buffer to treat cancer cell line. the citrate could be harmfull to cell if it is could I use same nanoparticles in PBS?

Various reason for the shift in plasmon peak position. The main reason is size. If you place the colloids in ambient condition, you will see settlement of black particles in the bottom because of slow aggregation. You are clearly explained what method you follow to synthesis gold nanoparticles. What type of capping agent, concentration of capping agent, etc.

@maria:I havnt use TEM as we dont have in our institute... I have done SEM and will be again doing it...

@nilufer: yes u can absolutely use PBS. I use PBS only for interactions of nanoparticles with biological samples

@nair: will certainly do that

2pandian: i am using green synthesis

hello All, Great writings! Could any of you help? I am looking for a good prep that will give me 10nm silver nanoparticles. Can anyone help? I am reducing AgNO3 using Sodium borohydride in the presence of citrate. I have tried numerous preps and seem to obtain non spherical globular particles which have a good UV peak (approx 420) but poor quality DLS (PDI >0.2). Can anyone help me please? I have gold well done. I can provide some gold in any size in PBS buffer in exchange for help with the silver protocol. Thank you!

The most useful results of Au or Ag NPs preparation could be obtained by Turkevich method (with some variations). But it is very important to work with Ag in dark place and water you use have to be deionized or distilled twice.

Thank you Maria. Would you happen to have a prep that you could recommend?

The main problem with that method is stability. After months they start to precipitate.

What is the most precise way off getting monodispersed silver nanoparticles that are stable? Can anyone help please?

The way I see it, from day 1 to day 20 the chemical reactions are still on but the gold ions concentration become low and more smaller particles formed. This is a possible the answer, why the absorbance maxima at 586 nm blue shifted to 577 nm.

@swati: i am using surfactant for my preparation as a stabilizer... Thank you very much for the paper reference...

@Elias Saion: I was carrying out the synthesis at 100degree celsius. but now it is stored in room temperature and it has been one month since its synthesis...still everyday i find a blue shift of about 0.5nm.

@all: Now I have started doing DLS analysis and hope that can answer some questions...Cannot comment anything by SEM as the concentration of auric chloride was very very low in the whole mixture...But I am getting good crystalline peaks from XRD after 30 days of preparation of the nanoparicles and its colour has started to become dark blue (slightly red) from light black in one month...one reason which could have been is the settling of larger particles but i shake the mixture before doing any characterization so I think precipitation of larger particles is not the answer...

Hi Anjul,

Like in quantum dots, I subscribe quantum mechanical perspective on optical absorption of metal nanoparticles and see at another angle. Interaction of UV-vis with polymer is what we see in UV-vis interacts with molecules due to electronic transition from HOMO to LUMO. But this may go on for many years as in Mie theory. For you, RK Taylor has answered based on the present theory.

First of all absoprtion peak at 586nm means your particles size is greater than 600-700nm and after 20 days size/ parameters might have changed. It can be due to instrumentation and handling differences. Or we can also say particles get more compacted with time. There might be one another reason that you have not purified nanoparticles and some reductant/stablizer is still in solution and with time it is showing some effects.

Anjul, if you have a sufficiently high quality XRD pattern I can try to get the size distribution out of it via WPPM. This would at least confirm or exclude some of the suggestions

http://pubs.acs.org/doi/abs/10.1021/jp106436h

A blue shift is odd, as spr-maxima generally red-shifts with increasing size and aggregation. My guess is that you particles, being large, have substantial absorption and scattering cross sections (the extinction spectrum is the sum of these). Smaller particles tend to mostly absorb, while larger aggregates tend to mostly scatter. You may be in the happy medium in which the effects are fairly similar. As such, what I would guess is happening is that you are getting slight aggregation, which is tilting that curve towards the scattering side. Although this would usually be a redshift, do to the relative interplay between the absorption and scattering functions, we may be seeing weight shifted around such that the net extinction spectrum appears to give a blueshift. You can corroborate this if you look at the long wavelength region. Is there weight increase out here? If so, I bet this is what's happening.

It's hard to for me to believe your particles are shrinking. I've stored citrate-capped AuNP's (albeit 12-50nm diameters) for over a year without and indication that this is even possible.