You can also take UV absorption spectra of your nanoparticles powder without any solution. But if you want to use methanol, then increase the concentration and take spectra...like 2mg/ml, if not coming, increase to 5 mg/ml.....

If you have fabricated the iron nanoparticles yourself, its better to first check their size through Zetasizer. Sometime UV scan can be problematic due to size issue. To get a size within the range of 50-200nm, its better to perform Ultransonication after the fabrication and then analyze the size(for verifivcation0.

For Ultransonication, make a solution of nearly 0.1-0.5wt% of the fabricated powder in 500ml water and perform the Ultransonication for 25mins in 2 cycles and throw the half of the solution and use the remaining half of the solution for UV spectroscopy. Fill the cuvettes up to nearly half of the size and perform the calibration before starting UV scan

Good luck with your 'dispersion' of a dry dense 'nanopowder' in a thin solvent.....

Also, try a Stokes'Law calculation of the size of Fe (density ~ 7.5 g/cm3) that would allow it to remain in free (colloidal) suspension.

thank you for answering. Can I use ethanol as solvent if methanol is not available? and in case of buffer which buffer should I use?

You need powder sample which should be dissolved or suspended in solvents to record UV-Vis Spectrum of iron nano-particles. As far we know the absorption peak of iron nano-particles will be observed at 261nm. In case of buffer, you can disperse iron nanoparticles in PBS buffer of pH 7.2.