First of all if your colloid is stable enough then you can do emission studies at any time, if not, then record emission spectra soon after synthesizing the material.

Here it is not clear which material you have synthesized, hence it is difficult to suggest whether LED with 940 nm will be enough to record emission or not. However, you can record UV-Vis spectra very easily as it water soluble and from the band edge you can find out the band-gap of the material, none other than the wavelength of emission. 

it is rather difficult to comment without knowing the type of nano particles. Also 940nm falls in IR category. (completely invisible to naked eyes) yess u do get IR LED check out websites like element 14 or Edmund optics. ( if u have an old TV remote remove the LED it is 808 nm check with that for initial study). Emission can be done immediately as suggested by Sujoy.

Are you synthesizing quantum dots ?

NaYF4:Yb/Tm. They absorb at 980 nm and emit in vis and UV range.

Dear Aastha,

If your synthesized material contains d or f block elements, then theoretically you can expect emission with your LED as excitation source..

i agree, you can use LED for excitation source.

measuring the emission would be a big task, can you tweek your PL machine to obtain a quantitative data.

Thank you sir

Your UCNPs was NaYF4:Yb/Tm, so it is best way to use 975-980 nm source to test upconversion emissions, you know it has a big absorption cross section for Yb3+. 

Probably the excitation power of a simple LED is not enough to excite the UCNPs. If it doesn't work I would suggest a continuous wave laser at 980 nm.

Check these links from where you can purchase the laser 

http://www.roithner-laser.com/laser_pointers_980.html 

http://www.ebay.com/bhp/940nm-ledAdd your answer

Most upconversion nanoparticles are also power-dependent emission type of nanoparticles under light excitation. If you do not provide enough power to excite the nanoparticles, you will not get correct emission spectra. Therefore, people prefer to use lasers at early days, because at that time no high power LED were commercial available. However, if you will develop upconversion nanoparticles for a long run, an adjustable power laser (up to 2W) is the number one choice. You can buy it from China for ~US$2000, maybe even cheaper now. The only problem for this kind of laser from China is stability. Do not run at high power mode for a long time.

Maria Elena Materia, Yen-jun Chuang you were right about LED. I tried LED (2W) but could not see the emission.

Then I tried Laser (180mW), and I got perfect emission which was visible with naked eye.

Thank you all for your help

Hi Aastha, with 100mW diode laser you can get beautiful upconversion emission from your sample. but the sample preparation and recording the up-conversion emission is tricky.. You can go though our work for a better understanding regarding the mechanism of upconversion in capped nanosystems...

Hi Aastha and Others,

any updates on this topic/question?

Thanks

UC emission is greatly depend on the excitation power density (irradiance). Lasers, by nature, have irradiance that helps to produce measurable UC emission. Whereas LEDs have very large divergence angle (~120 degrees). Therefore, the flux decreases exponentially with the increase in the distance from the LED. A combination of high quantum yield UC crystals, collimated LEDs, and a sensitive detection system could help to achieve UC imaging with LEDs. You could find more details in the below ACS abstract. The paper provides information about quantum yield measurement of UC crystals.

Acquisition of upconversion luminescence images exciting with LEDs: A paradigm shift in upconversion imaging

A Baride, J Meruga, W Cross, J Kellar, S May

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 255

Measuring the internal quantum yield of upconversion luminescence for ytterbium-sensitized upconversion phosphors using the ytterbium (iii) emission as an internal standard

PS May, A Baride, MY Hossan, M Berry

Nanoscale 10 (36), 17212-17226

Thanks