Applying UV-Vis spectroscopy to analyze nano particlescos

UV-Vis technique is a required technique to study the optics of materials. Optical research helps researchers apply nanomaterials for different uses. Various devices are built based on the optical properties of materials, especially solar cells. With the help of this technique, the optical band gap can be calculated, which helps in selecting materials for the purpose of converting energy such as light energy into electrical energy in solar cells. Additionally, from this technique, the purity of the sample can be measured by comparison with a reference solution.

Principle of UV-Vis spectroscopy

Ultraviolet and visible absorption spectroscopy (UV-Vis) is a technique that measures the attenuation of light transmitted through a sample or reflected from it. When light shines on the sample, each material absorbs a specific range of light and exhibits corresponding behavior. This follows the principle of Beer Lambert's law, which states that the absorption of light by a sample is proportional to the path length and concentration of the sample.

Mathematically,

A = log (I0/I) = εcl

Where,

A = Absorbance

I0 = Light intensity shining on the sample cell

I = Intensity of light leaving the sample cell

C = Molar concentration of solute

L = Length of sample cuvette (cm.)

ε = Molar absorptivity

Application of UV-Vis spectroscopy

The most important element of this technique is its application to the researcher to provide information about the material when light shines on it.

Chromophore Functional Group Detection: To individually identify the functional group in the material, it confirms the presence and absence of the Functional Group in the sample which must be compound. Chromophore is an atom or group responsible for the color of a compound.

Identification of unknown compounds: With the help of UV-Vis spectroscopy, unknown compounds can be identified in the sample. For this purpose, the required compound is compared with the spectrum of the reference compound, if luckily both spectra match then confirmation of the unknown compound can be recorded.

Sample Purity: Substance purity can be measured using this unique technique for the purpose of comparing the absorbance of a reference and observed sample and through relative calculations of purity The absorption intensity of the sample can be confirmed.

Size of quantum dots: Quantum dots are especially interesting when it comes to UV-vis spectroscopy because the size of the quantum dot can be determined from the position of the absorption peak in the UV-vis spectrum. Quantum dots absorb different wavelengths depending on the size of the particles. Many calibration curves would need to be done to determine the exact size and concentration of the quantum dots, but it is entirely possible and very useful to be able to determine size and concentration of quantum dots in this way since other ways of determining size are much more expensive and extensive (electron microscopy is most widely used for this data).

Figure. Absorbance of different sized CdSe QDs. Reprinted with permission from C. B. Murray, D. J. Norris, and M. G. Bawendi, J. Am. Chem. Soc., 1993, 115, 8706. Copyright: American Chemical Society (1993).

Ultraviolet-visible spectroscopy of noble metal nanoparticles: Noble metal nanoparticles have been used for centuries to color-stained glass windows and offer many opportunities for sensing and new optics due to their strong ability to scatter (deflect) and absorb light. One of the most interesting and important properties of noble metal nanoparticles is their localized surface plasmon resonance (LSPR). The LSPR of noble metal nanoparticles arises when photons of a certain frequency induce collective oscillation of conduction electrons on the nanoparticle surface. This causes selective photon absorption, effective scattering, and enhanced electromagnetic field strength around the nanoparticles. Ultraviolet-visible absorption spectroscopy is a powerful tool for the detection of noble metal nanoparticles, because the LSPR of metal nanoparticles allows highly selective absorption of photons. UV absorption spectroscopy can also be used to detect various factors affecting the LSPR of noble metal nanoparticles.

UV-Vis spectroscopy to predict nanoparticle geometry: nanoparticles’ UV-visible absorbance spectrum can be used to predict their geometry. As shown in Figure below, the UV-visible absorbance spectrum is highly dependent on nanoparticle geometry. The shapes of the two spectra are quite different despite the two types of nanoparticles having similar dimensions and being composed of the same material.

Figure. UV-visible absorbance spectra of 50 nm diameter gold nanospheres (A) and 25 nm diameter, 60 nm length gold nanorods (B).

The ultraviolet-visible spectrum determines the state of nanoparticle aggregation: the visible ultraviolet absorption spectrum also depends on the state of aggregation of the nanoparticles. When nanoparticles are close to each other, their plasmons couple to each other, which affects their LSPR and thus their light absorption. Aggregation of nanoparticles reduces the maximum absorption intensity without changing the wavelength at which the maximum occurs (λmax).

Figure. UV-visible absorbance spectrum of 50 nm gold nanosphere dimers with a reference spectrum of single gold nanospheres (A) and UV-visible absorbance spectrum of 50 nm gold nanospheres exposed to various concentrations of NaCl (B).

Band gap calculations: An interesting application of this technique is the calculations of the band via different method there are different software’s are available for this purpose.

Mathematically:

E = hc/λ (1)

Where,

E = Energy band gap

h = Planks constant

C = Speed of light

λ = Wavelength

INTINS has products from Ocean Optics (Ocean Insight) for this measurement such as Ocean HR Series, Ocean SR Series, Ocean FX. These are all excellent choices for high-speed process applications and measurement of fast events. They are also well-suited applications including laser characterization, plasma monitoring, and absorbance measurements, ranging from plasma monitoring to pharmaceuticals analysis, etc.