XPS Analysis of nanoparticle encapsulated Insulin

The oral intake of insulin is treated as an alternative to the widespread injection of this hormone. In this case biodegradable nanoparticles (NP) are used as an insulin carrier. The nanoparticles are covering the insulin and only release it after it has  reached the bloodstream. The quality of the nanoshell and the amount of the encapsulated hormone are determining factors in the effectivity of the oral intake. Therefore, surface sensitive XPS investigations were combined with volume analytical techniques to obtain qualitative and quantitative information about the insulin encapsulation.

In principle, the empty NP and the Insulin yield very similar XPS spectra with the main elements of C, O and N. However, insulin can be distinguished from the NP by its characteristic disulfide bridge (Cys-S-S-Cys) as the S2p detailed spectrum in the above picture shows. Because of the disulfide bridges the insulin yields signals with a S2p3/2 binding energy (BE) of 163.5 eV. The SO32- / SO42- bonds that are also detected (at a BE of about 168 eV) are due to a sulfur component from the NP.

Because of its information depth, it is possible to perform a quantitative analysis of insulin in the top 10 nm of the nanoparticle. A comparison with the results of a determination of the total insulin content thus enables an estimate of the surface portion of the hormone.

NP batch

Insulin in NP

Insulin at surface

#1

13.60 % 5.00 %
#1, rinsed 3.60 %
#2 13.30 % 21.00 %
#2, rinsed 5.30 %

The table shows an example of results of two NP-batches. Though the total insulin content of both batches is almost identical, the XPS data show a significantly higher proportion of insulin on the surface of batch 2. In addition, the insulin content of this batch can be significantly reduced by washing. This indicates an incorrect or incomplete encapsulation of the hormone. Because of this result, the corresponding NPs are unsuitable as a carrier of insulin for oral intake.