Mitochondrial electron transport chain in macrophage reprogramming: Potential role in antibacterial immune response.

Macrophages restrain microbial infection and reinstate tissue homeostasis. The mitochondria govern macrophage metabolism and serve as pivot in innate immunity, thus acting as immunometabolic regulon. Metabolic pathways produce electron flows that end up in mitochondrial electron transport chain (mtETC), made of super-complexes regulating multitude of molecular and biochemical processes. Cell-intrinsic and extrinsic factors influence mtETC structure and function, impacting several aspects of macrophage immunity. These factors provide the macrophages with alternate fuel sources and metabolites, critical to gain functional competence and overcoming pathogenic stress. Mitochondrial reactive oxygen species (mtROS) and oxidative phosphorylation (OXPHOS) generated through the mtETC are important innate immune attributes, which help macrophages in mounting antibacterial responses. Recent studies have demonstrated the role of mtETC in governing mitochondrial dynamics and macrophage polarization (M1/M2). M1 macrophages are important for containing bacterial pathogens and M2 macrophages promote tissue repair and wound healing. Thus, mitochondrial bioenergetics and metabolism are intimately coupled with innate immunity. In this review, we have addressed mtETC function as innate rheostats that regulate macrophage reprogramming and innate immune responses. Advancement in this field encourages further exploration and provides potential novel macrophage-based therapeutic targets to control unsolicited inflammation.

ß-Catenin Elicits Drp1-Mediated Mitochondrial Fission Activating the Pro-Apoptotic Caspase-1/IL-1ß Signalosome in Aeromonas hydrophila-Infected Zebrafish Macrophages.

Canonical Wnt signaling plays a major role in regulating microbial pathogenesis. However, to date, its involvement in A. hydrophila infection is not well known. Using zebrafish (Danio rerio) kidney macrophages (ZKM), we report that A. hydrophila infection upregulates wnt2, wnt3a, fzd5, lrp6, and ß-catenin (ctnnb1) expression, coinciding with the decreased expression of gsk3b and axin. Additionally, increased nuclear ß-catenin protein accumulation was observed in infected ZKM, thereby suggesting the activation of canonical Wnt signaling in A. hydrophila infection. Our studies with the ß-catenin specific inhibitor JW67 demonstrated ß-catenin to be pro-apoptotic, which initiates the apoptosis of A. hydrophila-infected ZKM. ß-catenin induces NADPH oxidase (NOX)-mediated ROS production, which orchestrates sustained mitochondrial ROS (mtROS) generation in the infected ZKM. Elevated mtROS favors the dissipation of the mitochondrial membrane potential (ΔΨm) and downstream Drp1-mediated mitochondrial fission, leading to cytochrome c release. We also report that ß-catenin-induced mitochondrial fission is an upstream regulator of the caspase-1/IL-1ß signalosome, which triggers the caspase-3 mediated apoptosis of the ZKM as well as A. hydrophila clearance. This is the first study suggesting a host-centric role of canonical Wnt signaling pathway in A. hydrophila pathogenesis wherein ß-catenin plays a primal role in activating the mitochondrial fission machinery, which actively promotes ZKM apoptosis and helps in containing the bacteria.

Role of UPRmt and mitochondrial dynamics in host immunity: it takes two to tango.

The immune system of a host contains a group of heterogeneous cells with the prime aim of restraining pathogenic infection and maintaining homeostasis. Recent reports have proved that the various subtypes of immune cells exploit distinct metabolic programs for their functioning. Mitochondria are central signaling organelles regulating a range of cellular activities including metabolic reprogramming and immune homeostasis which eventually decree the immunological fate of the host under pathogenic stress. Emerging evidence suggests that following bacterial infection, innate immune cells undergo profound metabolic switching to restrain and countervail the bacterial pathogens, promote inflammation and restore tissue homeostasis. On the other hand, bacterial pathogens affect mitochondrial structure and functions to evade host immunity and influence their intracellular survival. Mitochondria employ several mechanisms to overcome bacterial stress of which mitochondrial UPR (UPRmt) and mitochondrial dynamics are critical. This review discusses the latest advances in our understanding of the immune functions of mitochondria against bacterial infection, particularly the mechanisms of mitochondrial UPRmt and mitochondrial dynamics and their involvement in host immunity.

TLR22-Induced Pro-Apoptotic mtROS Abets UPRmt-Mediated Mitochondrial Fission in Aeromonas hydrophila-Infected Headkidney Macrophages of Clarias gariepinus.

Toll-like receptors (TLRs) are epitomized as the first line of defense against pathogens. Amongst TLRs, TLR22 is expressed in non-mammalian aquatic vertebrates, including fish. Using headkidney macrophages (HKM) of Clarias gariepinus, we reported the pro-apoptotic and microbicidal role of TLR22 in Aeromonas hydrophila infection. Mitochondria act as a central scaffold in the innate immune system. However, the precise molecular mechanisms underlying TLR22 signaling and mitochondrial involvement in A. hydrophila-pathogenesis remain unexplored in fish. The aim of the present study was to investigate the nexus between TLR22 and mitochondria in pro-apoptotic immune signaling circuitry in A. hydrophila-infected HKM. We report that TLR22-induced mitochondrial-Ca2+ [Ca2+]mt surge is imperative for mtROS production in A. hydrophila-infected HKM. Mitigating mtROS production enhanced intracellular bacterial replication implicating its anti-microbial role in A. hydrophila-pathogenesis. Enhanced mtROS triggers hif1a expression leading to prolonged chop expression. CHOP prompts mitochondrial unfolded protein response (UPRmt) leading to the enhanced expression of mitochondrial fission marker dnml1, implicating mitochondrial fission in A. hydrophila pathogenesis. Inhibition of mitochondrial fission reduced HKM apoptosis and increased the bacterial burden. Additionally, TLR22-mediated alterations in mitochondrial architecture impair mitochondrial function (ΔΨm loss and cytosolic accumulation of cyt c), which in turn activates caspase-9/caspase-3 axis in A. hydrophila-infected HKM. Based on these findings we conclude that TLR22 prompts mtROS generation, which activates the HIF-1α/CHOP signalosome triggering UPRmt-induced mitochondrial fragmentation culminating in caspase-9/-3-mediated HKM apoptosis and bacterial clearance.

Th1-Th2 and M1-M2 interplay sculpt Aeromonas hydrophila pathogenesis in zebrafish (Danio rerio).

Aeromonas hydrophila is an important aquatic zoonotic pathogen that causes septicemia, necrotizing fasciitis and gastroenteritis in various aquatic and non-aquatic animals. However, the pathogenesis of A. hydrophila is not fully understood. Here, we examined the pathogenicity and histopathology of A. hydrophila in the zebrafish (Danio rerio) model system. We found that the intensity of symptoms and mortality is dose-dependent. Bacterial colonization studies demonstrated that A. hydrophila never cleared out from the fish body but stayed in a state of inactivity till it enters a fresh host. Reinfection studies showed that exposure to A. hydrophila provides immunity against future infection and hence improves fish survival. Gene expression studies revealed the crosstalk between T-helper cell and macrophage responses in fish immune system in response to A. hydrophila and infection memory. Histopathological studies showed that symptoms of tissue damage and inflammation lasted for less duration with less intensity in immunized fish when compared to non-immunized fish. Together, our results suggest that the zebrafish model is a useful system in studying the interplay between A. hydrophila pathogenesis, persistence and immunity.

Aeromonas hydrophila inhibits autophagy triggering cytosolic translocation of mtDNA which activates the pro-apoptotic caspase-1/IL-1ß-nitric oxide axis in headkidney macrophages.

The molecular mechanisms underlying Aeromonas hydrophila-pathogenesis are not well understood. Using head kidney macrophages (HKM) of Clarias gariepinus, we previously reported the role of ER-stress in A. hydrophila-induced pathogenesis. Here, we report that PI3K/PLC-induced cytosolic-Ca2+ imbalance induces the expression of pro-apoptotic ER-stress marker, CHOP in A. hydrophila-infected HKM. CHOP promotes HKM apoptosis by inhibiting AKT activation and enhancing JNK signaling. Elevated mitochondrial ROS (mtROS) was recorded which declined significantly by ameliorating ER-stress and in the presence of ER-Ca2+ release modulators (2-APB and dantrolene) and mitochondrial-Ca2+ uptake inhibitor, Ru360, together suggesting the role of ER-mitochondrial Ca2+ dynamics in mtROS generation. Inhibiting mtROS production reduced HKM death implicating the pro-apoptotic role of mtROS in A. hydrophila-pathogenesis. The expression of autophagic proteins (LC3B, beclin-1, and atg 5) was suppressed in the infected HKM. Our results with autophagy-inducer rapamycin demonstrated that impaired autophagy favored the cytosolic accumulation of mitochondrial DNA (mtDNA) and the process depended on mtROS levels. Enhanced caspase-1 activity and IL-1ß production was detected and transfection studies coupled with pharmacological inhibitors implicated mtROS/mtDNA axis to be crucial for activating the caspase-1/IL-1ß cascade in infected HKM. RNAi studies further suggested the involvement of IL-1ß in generating pro-apoptotic NO in A. hydrophila-infected HKM. Our study suggests a novel role of ER-mitochondria cross-talk in regulating A. hydrophila pathogenesis. Based on our observations, we conclude that A. hydrophila induces ER-stress and inhibits mitophagy resulting in mitochondrial dysfunction which leads to mtROS production and translocation of mtDNA into cytosol triggering the activation of caspase-1/IL-1ß-mediated NO production, culminating in HKM apoptosis.

TLR22-mediated activation of TNF-α-caspase-1/IL-1ß inflammatory axis leads to apoptosis of Aeromonas hydrophila-infected macrophages.

Toll-like receptors (TLRs) represent first line of host defence against microbes. Amongst different TLRs, TLR22 is exclusively expressed in non-mammalian vertebrates, including fish. The precise role of TLR22 in fish-immunity remains abstruse. Herein, we used headkidney macrophages (HKM) from Clarias gariepinus and deciphered its role in fish-immunity. Highest tlr22 expression was observed in the immunocompetent organ - headkidney; nonetheless expression in other tissues suggests its possible involvement in non-immune sites also. Aeromonas hydrophila infection up-regulates tlr22 expression in HKM. Our RNAi based study suggested TLR22 restricts intracellular survival of A. hydrophila. Inhibitor and RNAi studies further implicated TLR22 induces pro-inflammatory cytokines TNF-α and IL-1ß. We observed heightened caspase-1 activity and our results suggest the role of TLR22 in activating TNF-α/caspase-1/IL-1ß cascade leading to caspase-3 mediated apoptosis of A. hydrophila-infected HKM. We conclude, TLR22 plays critical role in immune-surveillance and triggers pro-inflammatory cytokines leading to caspase mediated HKM apoptosis and pathogen clearance.

M. fortuitum-induced CNS-pathology: Deciphering the role of canonical Wnt signaling, blood brain barrier components and cytokines.

Molecular underpinning of mycobacteria-induced CNS-pathology is not well understood. In the present study, zebrafish were infected with Mycobacterium fortuitum and the prognosis of CNS-pathogenesis studied. We observed M. fortuitum triggers extensive brain-pathology. Evans blue extravasation demonstrated compromised blood-brain barrier (BBB) integrity. Further, decreased expression in tight-junction (TJ) and adherens junction complex (AJC) genes were noted in infected brain. Wnt-signaling has emerged as a major player in host-mycobacterial immunity but its involvement/role in brain-infection is not well studied. Sustained expression of wnt2, wnt3a, fzd5, lrp5/6 and ß-catenin, with concordant decline in degradation complex components axin, gsk3ß and ß-catenin regulator capn2a were observed. The surge in ifng1 and tnfa expression preceding il10 and il4 suggested cytokine-interplay critical in M. fortuitum-induced brain-pathology. Therefore, we suggest adult zebrafish as a viable model for studying CNS-pathology and using the same, conclude that M. fortuitum infection is associated with repressed TJ-AJC gene expression and compromised BBB permeability. Our results implicate Wnt/ß-catenin pathway in M. fortuitum-induced CNS-pathology wherein Th1-type signals facilitate bacterial clearance and Th2-type signals prevent the disease sequel.

TLR-2 mediated cytosolic-Ca2+ surge activates ER-stress-superoxide-NO signalosome augmenting TNF-α production leading to apoptosis of Mycobacterium smegmatis-infected fish macrophages.

The implications of TLR-2 mediated alterations in cytosolic-Ca2+((Ca2+)c) levels in M. smegmatis infections is not well known. Using headkidney macrophages (HKM) from Clarias gariepinus, we observed TLR-2 signalling is required in the phagocytosis of M. smegmatis. M. smegmatis induced caspase-dependent HKM apoptosis in MOI, time and growth-phase dependent manner. RNAi and inhibitor studies demonstrated critical role of TLR-2 in eliciting (Ca2+)c-surge and c-Src-PI3K-PLC axis playing an intermediary role in the process. The (Ca2+)c-surge triggered downstream ER-stress and superoxide (O2-) generation. The cross-talk between ER-stress and O2- amplified TNF-α production, which led to HKM apoptosis and bacterial clearance. Release of nitric oxide (NO) was also observed and silencing the NOS2-NO axis enhanced intracellular bacterial survival and attenuated caspase activity. Pre-treatment with diphenyleneidonium chloride inhibited NO production implicating O2--NO axis imperative in M. smegmatis-induced HKM apoptosis. NO positively impacted CHOP expression and TNF-α production in infected HKM. We conclude that, TLR-2 induced (Ca2+)c-surge and ensuing cross-talk between ER-stress and O2- potentiates HKM pathology by amplifying pro-inflammatory TNF-α production. Moreover, the pro-oxidant environment triggers NO release which prolonged ER-stress and TNF-α production, culminating in HKM apoptosis and bacterial clearance. Together, our study suggests HKM an alternate model to study macrophage-mycobacteria interactions.

Tryptophan-kynurenine pathway attenuates ß-catenin-dependent pro-parasitic role of STING-TICAM2-IRF3-IDO1 signalosome in Toxoplasma gondii infection.

Recent studies have documented the diverse role of host immunity in infection by the protozoan parasite, Toxoplasma gondii. However, the contribution of the ß-catenin pathway in this process has not been explored. Here, we show that AKT-mediated phosphorylated ß-catenin supports T. gondii multiplication which is arrested in the deficiency of its phosphorylation domain at S552 position. The ß-catenin-TCF4 protein complex binds to the promoter region of IRF3 gene and initiates its transcription, which was also abrogated in ß-catenin knockout cells. TBK-independent phosphorylation of STING(S366) and its adaptor molecule TICAM2 by phospho-AKT(T308S473) augmented downstream IRF3-dependent IDO1 transcription, which was also dependent on ß-catenin. But, proteasomal degradation of IDO1 by its tyrosine phosphorylation (at Y115 and Y253) favoured parasite replication. In absence of IDO1, tryptophan was catabolized into melatonin, which supressed cellular reactive oxygen species (ROS) and boosted parasite growth. Conversely, when tyrosine phosphorylation was abolished by phosphosite mutations, IDO1 escaped its ubiquitin-mediated proteasomal degradation system (UPS) and the stable IDO1 prevented parasite replication by kynurenine synthesis. We propose that T. gondii selectively utilizes tryptophan to produce the antioxidant, melatonin, thus prolonging the survival of infected cells through functional AKT and ß-catenin activity for better parasite replication. Stable IDO1 in the presence of IFN-γ catabolized tryptophan into kynurenine, promoting cell death by suppressing phospho-AKT and phospho-ß-catenin levels, and circumvented parasite replication. Treatment of infected cells with kynurenine or its analogue, teriflunomide suppressed kinase activity of AKT, and phosphorylation of ß-catenin triggering caspase-3 dependent apoptosis of infected cells to inhibit parasite growth. Our results demonstrate that ß-catenin regulate phosphorylated STING-TICAM2-IRF3-IDO1 signalosome for a cell-intrinsic pro-parasitic role. We propose that the downstream IRF3-IDO1-reliant tryptophan catabolites and their analogues can act as effective immunotherapeutic molecules to control T. gondii replication by impairing the AKT and ß-catenin axis.

Aeromonas hydrophila utilizes TLR4 topology for synchronous activation of MyD88 and TRIF to orchestrate anti-inflammatory responses in zebrafish.

Toll-like receptor 4 (TLR4) plays a critical role in host immunity against Gram-negative bacteria. It transduces signals through two distinct TIR-domain-containing adaptors, MyD88 and TRIF, which function at the plasma membrane and endosomes, respectively. Using zebrafish Aeromonas hydrophila infection model, we demonstrate that synchronization of MyD88 and TRIF dependent pathways is critical for determining the fate of infection. Zebrafish were infected with A. hydrophila, and bacterial recovery studies suggested its effective persistence inside the host. Histopathological assessment elucidates that A. hydrophila did not provoke inflammatory responses in the spleen. Immunofluorescence revealed the presence of TLR4-bound A. hydrophila on the plasma membrane at 3 h post-infection (p.i.), and inside endosomes 1 day p.i. Quantitative PCR studies suggest that TLR4 activates the downstream pathway of MyD88-IRAK4 axis at early stages followed by a shift to TRIF-TRAF6 axis at late stages of infection coupled with fold increase in NFκB. Our results implicated the involvement of p110δ isoform of PI(3)Kinase in this transition. Coupled to this, we noted that the TLR4-TRIF-NFκB axis prompted burgeoned secretion of anti-inflammatory cytokines. We observed that A. hydrophila inhibits endosome maturation and escapes to cytoplasm. Significant downregulation of cytosolic-NLR receptors further suggested that A. hydrophila represses pro-inflammatory responses in cytosol aiding its persistence. Our findings suggest a novel role of 'TLR4 topology' in A. hydrophila-induced pathogenesis. We propose that A. hydrophila manipulates translocation of TLR4 and migrates to endosome, where it triggers TRIF-dependent anti-inflammatory responses, interferes with endosomal maturation and escapes to cytosol. Inside the cytosol, A. hydrophila avoids detection by suppressing NLRs, facilitating its survival and ensuing pathogenesis.

Chronic fluoride exposure exacerbates headkidney pathology and causes immune commotion in Clarias gariepinus.

The current study was aimed to understand the effects of chronic fluoride exposure on fish immune system. African sharp tooth catfish (Clarias gariepinus) were exposed to 73.45mg/L of fluoride corresponding to 1/10 96h LC50 for 30 d and the effects on general fish health and several immune parameters were studied. Chronic fluoride exposure led to significant alteration in serum biochemical parameters including alkaline phosphatase, alanine transaminase, aspartate transaminase, triglycerides, cholesterol and blood urea nitrogen levels revealing the detrimental effect of fluoride on general fish health. Upregulation in cytochrome P450 1A expression, both at mRNA and protein level suggested that fluoride activates the detoxification machinery in headkidney (HK) of C. gariepinus. Histopathological analysis of HK from exposed fish further revealed fluoride-induced hypertrophy, increase in melano-macrophage centers (MMCs) and the development of cell-depleted regions. Fluoride reduced headkidney somatic index (HKSI) and the phagocytic potential of headkidney macrophages (HKM). It induced caspase-3-dependent headkidney leukocyte (HKL) apoptosis, elevated superoxide generation and production of pro-inflammatory cytokine TNF-α besides suppressed T-cell proliferation in the exposed fish. We surmise the elevation in superoxide levels coupled with increased TNF-α production to be plausible causes of fluoride-induced HKL apoptosis. It is concluded that chronic fluoride exposure induces structure-function alterations in HK, the primary lymphoid organ in fish leading to impairment in immune responses.

Glycopolymeric gel stabilized N-succinyl chitosan beads for controlled doxorubicin delivery.

Here we report the synthesis and study of N-succinyl chitosan based hydrogel beads, stabilized with glycopolymeric network (NSC/Glc-gel) for application in anticancer drug delivery of doxorubicin (DOX). The bio-recognition of lectins by NSC/Glc-gel bead was also studied by UV-vis spectrophotometry. The beads were characterized using FT-IR, SEM and Thermogravimetric analysis. The extent of DOX loading was proportional to the degree of succinylation and the swelling kinetics of the beads showed pH dependency. The beads exhibited sustained release of DOX over a period of more than 15 days in an acidic pH, mimicking the microenvironment of tumor cells, and even lesser release at physiological pH. Release exponent 'n' derived from Korsmeyer-Peppas model implied that NSC88/Glc-gel (88% succinylation of chitosan) beads followed fickian diffusion controlled release mechanism whereas NSC75/Glc-gel (75% succinylation of chitosan) beads follow zero order release profile. The synthesized beads also displayed specificity to lectin Concanavalin A.

Removal of americium from aqueous nitrate solutions by sorption onto PC88A-impregnated macroporous polymeric beads.

The removal of Am (III) ions from aqueous solutions was studied by solid-liquid extraction using indigenously synthesized Extractant Impregnated Macroporous Polymeric Beads (EIMPBs). These beads were prepared by an in situ phase inversion method using polyethersulfone (PES) as base polymer and 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A) as an extractant. The synthesized EIMPBs were characterized by FTIR, TGA and SEM techniques. The batch equilibration study using these beads for the uptake of Am (III) was carried out as a function of parameters, like pH, equilibration time, Am (III) concentration, etc. The blank polymeric beads, without PC88A, have shown negligible sorption of Am (III) under the experimental conditions. The experimental data on the sorption behavior of Am (III) on the polymeric beads fitted well in the pseudo-second-order kinetics model. The synthesized polymeric beads exhibited very good sorption capacity for Am (III) at pH 3. The reusability of the beads was also ascertained by repetitive sorption/desorption of Am (III) up to 10 cycles of operation, without any significant change in their sorption characteristics.

Silver nanoparticle-loaded PVA/gum acacia hydrogel: synthesis, characterization and antibacterial study.

A simple one-pot method for in situ synthesis of silver nanoparticles (AgNPs), within polyvinyl alcohol/gum acacia (PVA-GA) hydrogel matrix, by gamma radiation-induced cross-linking is reported here. The synthesized hydrogels were characterized by FT-IR, thermogravimetry, dynamic light scattering and inductively coupled mass spectrometry method. The thermal stability was found to be more for the hydrogel loaded with silver nanoparticles and also the percentage silver loading was found to increase with increase in cross-linking density. The influence of gum acacia (GA) concentration on the equilibrium degree of swelling of the synthesized hydrogels, and also on the silver release from hydrogel matrix, was investigated. The size of the silver nanoparticles formed in the hydrogel matrix was in the range of 10-40 nm. The rheological gel point was found to be at 25.34 kGy of radiation dose, for a typical hydrogel synthesized, using 5% GA, 3% PVA and 1mM AgNO3. The antibacterial studies of the synthesized nanosilver-containing hydrogels showed good antibacterial activity against gram-negative bacterium, Escherichia coli.

Dilution induced thickening in hydrotrope-rich rod-like micelles.

Dilution induced changes in the microstructure and rheological behavior of micelles formed by a cationic surfactant-anionic hydrotrope mixture has been investigated in the hydrotrope-rich region. The surfactant used is cetyltrimethylammonium bromide (CTAB) and the hydrotropic salt is sodium 3-hydroxy naphthalene 2-carboxylate (SHNC). The concentration of the mixture is varied from 0.5% to 10.0% w/w (φ=0.005-0.100) at a fixed weight ratio of hydrotrope to surfactant (85:15). Rheological studies indicate Newtonian flow behavior at low and high volume fractions (0.005 and 0.100) while a shear thinning behavior is observed at intermediate volume fractions. The zero-shear viscosity η(0) also passes through a maximum upon changes in the concentration. The most striking feature in our study is that a low viscosity Newtonian fluid transforms to a viscoelastic fluid, upon dilution, and then again to a Newtonain fluid. Small angle neutron scattering studies of 10.0% micellar solution show the presence of rod-like aggregates. Upon dilution, the scattering intensity per unit concentration shows an increase in the low q-region. The nature of pair distance distribution function and subsequent model fitting indicates a transition from rod-like micelles to unilamellar vesicles upon dilution. This behavior is explained in terms of the volume fraction dependant solubilization of hydrotropes in the rod-like micelles and consequent changes in the composition of the mixed micelles.

Precursor concentration and temperature controlled formation of polyvinyl alcohol-capped CdSe-quantum dots.

Polyvinyl alcohol-capped CdSe quantum dots, with a size within their quantum confinement limit, were prepared in aqueous solution at room temperature, by a simple and environmentally friendly chemical method. The size of the CdSe quantum dots was found to be dependent on the concentrations of the precursors of cadmium and selenium ions, as well as on the aging time and the reaction temperature; all of which could be used conveniently for tuning the size of the particles, as well as their optical properties. The synthesized quantum dots were characterized by optical absorption spectroscopy, fluorescence spectroscopy, X-ray diffraction, atomic force microscopy and transmission electron microscopy. The samples were fluorescent at room temperature; the green fluorescence was assigned to band edge emission, and the near-infrared fluorescence peaks at about 665 and 865 nm were assigned to shallow and deep trap states emissions, respectively. The quantum dots were fairly stable up to several days.

EPR investigations on radiation induced chemical transformations in Pd(ClO4)2/i-PrOH/HClO4 system from 77 to 300 K.

The EPR studies have been carried out on gamma irradiated samples of Pd(ClO4)2 in 9 M/4M HClO4 in the presence and absence of isopropanol [i-PrOH (5%, v/v)], in the temperature range of 77-300 K. The EPR studies revealed the formation of Pd+ complex, Pd3+ complex and several radicals such as H., OH., ClO3., ClO4. and O3- on radiolysis of these systems. Reduction of Pd2+ to Pd clusters could not be observed in the absence of i-PrOH and warming of the irradiated sample finally resulted in recovery of Pd2+ complex. However, in the presence of 5% i-PrOH, strongly oxidizing radicals like OH., ClO3. and ClO4. are suppressed and (CH3)2C.OH radical was produced along with reducing species like H.. EPR spectra in these strongly reducing conditions showed formation of Pd+ complex, which on heating above 230 K, disproportionated into Pd0 and Pd2+ complex and finally lead to formation of Pd metal aggregates. Simultaneously, pulse radiolysis technique has been employed as an alternative method to verify the EPR observations.