Intrinsic giant magnetoresistance due to exchange-bias-type effects at the surface of single-crystalline NiS2 nanoflakes

The coexistence of different properties in the same material often results in exciting physical effects. At low temperatures, the pyrite transition-metal disulphide NiS₂ hosts both antiferromagnetic and weak ferromagnetic orders, along with surface metallicity dominating its electronic transport. The interplay between such a complex magnetic structure and surface-dominated conduction in NiS₂, however, is still not understood. A possible reason for this limited understanding is that NiS₂ has been available primarily in bulk single-crystal form, which makes it difficult to perform studies combining magnetometry and transport measurements with high spatial resolution. Here, NiS₂ nanoflakes are produced via mechanical cleaving and exfoliation of NiS₂ single crystals and their properties are studied on a local (micron-size) scale. Strongly field-asymmetric magnetotransport features are found at low temperatures, which resemble those of more complex magnetic thin film heterostructures. Using nitrogen vacancy magnetometry, these magnetotransport features are related to exchange-bias-type effects between ferromagnetic and antiferromagnetic regions forming near step edges at the nanoflake surface. Nanoflakes with bigger steps exhibit giant magnetoresistance, which suggests a strong influence of magnetic spin textures at the NiS₂ surface on its electronic transport. These findings pave the way for the application of NiS₂ nanoflakes in van der Waals heterostructures for low-temperature spintronics and superconducting spintronics.

[In vitro comparison of the photodynamic activity of meso-tetra (m-hydroxyphenyl) chlorin and hematoporphyrin derivative]

Meso-tetra (hydroxyphenyl) chlorin (mTHPC) was evaluated in vitro in HT29 human colon adenocarcinoma cell line and compared with hematoporphyrin derivative (HpD). The incorporation kinetics, evaluated by flow cytometry showed that mTHPC and HpD cellular uptake was related to the incubation time until 12 hours, then a plateau appeared. Cytotoxicity assays were performed using MTT test after photoirradiation at 650 nm for mTHPC and 630 nm for HpD. The photodynamic activity of both photosensitizers was influenced by serum protein of the culture medium and time interval between irradiation and photocytotoxicity test with a maximal activity 24 and 48 hrs after the photoirradiation. In optimized experimental conditions i.e. 2% serum protein- containing culture medium, 24 hr-incubation period, cytotoxicity measured 48 hrs after exposure, 10 J/cm2 light fluence, mTHPC appeared approximately 20-fold more active than HpD with IC50 values of 0.2 micrograms/mL and 4.2 micrograms/mL, respectively.

MTT assays allow quick and reliable measurement of the response of human tumour cells to photodynamic therapy

MCF-7 and HT-29 cell lines were selected as a reliable model to examine the possible parameters affecting the sensitivity of tumour cells to photodynamic therapy (PDT) using a dye-laser at 630 nm. The chemical composition of haematoporphyrin derivative (HPD) was determined by high-performance liquid chromatography (HPLC) analysis and was in agreement with reported values. MTT assays were performed to assess the time-dependency of PDT and the influence of the output power and light fluence. The results showed a maximal cytotoxicity 48 h after photoirradiation. The output power (1 or 2 W) did not significantly affect the cytotoxicity when the fluence was constant (20 J/cm2). However, an increase in fluence (10-40 J/cm2) led to a significant enhancement of cytotoxicity until maximal values were reached (30-40 J/cm2). A further increase in fluence (50 J/cm2) proved to induce a fall-off in cytotoxicity related to the intense photobleaching of HPD.