Understanding and engineering interfacial charge transfer of carbon nanotubes and graphene for energy and sensing applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references.Graphene is a one-atom thick planar monolayer of sp2 -bonded carbon atoms organized in a hexagonal crystal lattice. A single walled carbon nanotube (SWCNT) can be thought of as a graphene sheet rolled up into a seamless hollow cylinder with extremely high length-to-diameter ratio. Their large surface area, and exceptional optical, mechanical and electronic properties make these low-dimensional carbon materials ideal candidates for (opto-)electronic and sensing applications. In this thesis I studied the charge transfer processes that occur at their interface, and developed applications based on the discovered properties. When light is incident on a semiconducting SWCNT, it can excite an electron from the valence band to the conduction band, thereby creating a Coulombically bound electron-hole pair, also known as an exciton. Excitons can decay via radiative or non-radiative recombination or by colliding with other excitons. They can diffuse along the length of a SWCNT or hop from larger band gap SWCNTs to smaller band gap SWCNTs, a process known as exciton energy transfer (EET). We studied their behavior as a function of temperature in SWCNT fibers and showed that at room temperature the rate constant for EET is more than two orders of magnitude larger than that of each of the different recombination processes. This led us to construct a core-shell SWCNT fiber, which consists of a core of smaller band gap SWCNTs, surrounded by a shell of larger band gap SWCNTs, essentially forming what is known as a type I heterojunction. In agreement with a model that describes exciton behavior in the SWCNT fibers, we found that upon illumination all the energy (in the form of excitons) was quickly transferred from the shell to the core, faster than the excitons would otherwise recombine. The SWCNT fiber proved to be an efficient optical and energetic concentrator. We showed that SWCNTs and poly(3-hexylthiophene) (P3HT) form a type II heterojunction, which implies that excitons generated in the P3HT can easily dissociate into free charge carriers at the interface with the SWCNTs. Despite this, the efficiency of a P3HT/SWCNT bulk heterojunction (BHJ) photovoltaic is subpar. We developed a P3HT/SWCNT planar heterojunction (PHJ) and achieved efficiencies that were 30 times higher, which showed that the formation of bundled aggregates in BHJs was the cause: metallic SWCNTs can quench the excitons in an entire bundle. Another interesting feature of our SWCNT/P3HT PHJ is that a maximum efficiency was reached when -60 nm of P3HT was used, which is surprising since in a planar photovoltaic a maximum is expected for ~8.5 nm of P3HT, the value of the exciton diffusion length. A Kinetic Monte Carlo simulation revealed that bulk exciton dissociation was responsible for the lower efficiencies observed in devices with low P3HT thickness. Next we created and studied a junction between SWCNTs and a monolayer of graphene, an ideal one-dimensional/two-dimensional carbon interface. We used Raman spectroscopy to probe the degree of charge transfer at the interface and based on a shift in the G peak position of the graphene Raman signal at the junction deduced that a typical metallic (semiconducting) SWCNT dopes the graphene with 1.12 x 1013 cm-2 (0.325 x 101 cm-2) electrons upon contact, in agreement with the fact that the Fermi level of the SWCNTs is more shallow than that of the graphene. A molecular dynamics simulation ruled out that the observed Raman peak shifts are due to strain, although it did show that SWCNTs are being compressed radially by the graphene sheet, resulting in a widening of their Raman peaks. We studied charge transfer between diazonium molecules and graphene, to better inform transistor and sensor design. The reaction rate depends on the degree of overlap between the filled energy levels in graphene and the unoccupied ones in the diazonium molecule. We showed that with increasing degree of functionalization the charge transfer characteristics of a graphene field effect transistor (FET) alter in the following ways: the minimum conductivity decreases, the Dirac point upshifts, the conductivity plateau at high carrier density decreases and the electronhole conduction asymmetry increases. We developed a theoretical model of charge transport in graphene FETs that takes into account the effect of both short-range and long-range scatterers. Fitting it to the charge-transport data reveals quantitative information about the number of impurities in the substrate supporting the graphene, about the number of defects created as a result of the reaction, and about the degree of electron-hole conduction asymmetry. Graphene functionalization also affects the graphene Raman signal. After reaction, the D to G intensity ratio to increases, which is a sign of covalent modification of the graphene lattice. Additionally, the G peak and 2D peak positions increase while the 2D/G intensity ratio decreases, which are signs of hole-doping. Based on a Raman analysis, we were also able to show that the end group of the diazonium salt can affect both the degree of chemisorption (covalent modification) as well as the degree of physisorption (doping). Finally, we studied the effects of charge transfer between graphene and biological cells on the graphene Raman signal and designed a fundamentally new type of biosensor. Graphene can be thought of as a continuous array of information units (sensor units). The Raman signal collected in each unit can report on its local environment. In contrast to graphene FET biosensors, the graphene Raman biosensor offers subcellular spatial resolution. The graphene Raman signal was shown to display a strong dependence on pH. Metabolically active cells acidify their local environment; therefore, pH is a proxy for cellular metabolism. We placed both human embryonic kidney (HEK) cells that were genetically engineered to produce mouse antibodies and control HEK cells that were not genetically modified onto the graphene. Based on the change in the graphene Raman signal we deduced the former have a metabolic rate that is four times higher than that of the control cells. Increased cellular adhesion allows the cells to interact more closely with the graphene monolayer and intensifies the observed Raman effects.by Geraldine L.C. Paulus.Ph.D

Rare predicted loss-of-function variants of type I IFN immunity genes are associated with life-threatening COVID-19

BackgroundWe previously reported that impaired type I IFN activity, due to inborn errors of TLR3- and TLR7-dependent type I interferon (IFN) immunity or to autoantibodies against type I IFN, account for 15-20% of cases of life-threatening COVID-19 in unvaccinated patients. Therefore, the determinants of life-threatening COVID-19 remain to be identified in similar to 80% of cases.MethodsWe report here a genome-wide rare variant burden association analysis in 3269 unvaccinated patients with life-threatening COVID-19, and 1373 unvaccinated SARS-CoV-2-infected individuals without pneumonia. Among the 928 patients tested for autoantibodies against type I IFN, a quarter (234) were positive and were excluded.ResultsNo gene reached genome-wide significance. Under a recessive model, the most significant gene with at-risk variants was TLR7, with an OR of 27.68 (95%CI 1.5-528.7, P=1.1x10(-4)) for biochemically loss-of-function (bLOF) variants. We replicated the enrichment in rare predicted LOF (pLOF) variants at 13 influenza susceptibility loci involved in TLR3-dependent type I IFN immunity (OR=3.70[95%CI 1.3-8.2], P=2.1x10(-4)). This enrichment was further strengthened by (1) adding the recently reported TYK2 and TLR7 COVID-19 loci, particularly under a recessive model (OR=19.65[95%CI 2.1-2635.4], P=3.4x10(-3)), and (2) considering as pLOF branchpoint variants with potentially strong impacts on splicing among the 15 loci (OR=4.40[9%CI 2.3-8.4], P=7.7x10(-8)). Finally, the patients with pLOF/bLOF variants at these 15 loci were significantly younger (mean age [SD]=43.3 [20.3] years) than the other patients (56.0 [17.3] years; P=1.68x10(-5)).ConclusionsRare variants of TLR3- and TLR7-dependent type I IFN immunity genes can underlie life-threatening COVID-19, particularly with recessive inheritance, in patients under 60 years old

Effects of alirocumab on cardiovascular and metabolic outcomes after acute coronary syndrome in patients with or without diabetes: a prespecified analysis of the ODYSSEY OUTCOMES randomised controlled trial

Background After acute coronary syndrome, diabetes conveys an excess risk of ischaemic cardiovascular events. A reduction in mean LDL cholesterol to 1.4-1.8 mmol/L with ezetimibe or statins reduces cardiovascular events in patients with an acute coronary syndrome and diabetes. However, the efficacy and safety of further reduction in LDL cholesterol with an inhibitor of proprotein convertase subtilisin/kexin type 9 (PCSK9) after acute coronary syndrome is unknown. We aimed to explore this issue in a prespecified analysis of the ODYSSEY OUTCOMES trial of the PCSK9 inhibitor alirocumab, assessing its effects on cardiovascular outcomes by baseline glycaemic status, while also assessing its effects on glycaemic measures including risk of new-onset diabetes

Apolipoprotein B, Residual Cardiovascular Risk After Acute Coronary Syndrome, and Effects of Alirocumab.

Background: Apolipoprotein B (apoB) provides an integrated measure of atherogenic risk. Whether apoB levels and apoB lowering hold incremental predictive information on residual risk after acute coronary syndrome beyond that provided by low-density lipoprotein cholesterol is uncertain. Methods: The ODYSSEY OUTCOMES trial (Evaluation of Cardiovascular Outcomes After an Acute Coronary Syndrome During Treatment With Alirocumab) compared the proprotein convertase subtilisin/kexin type 9 inhibitor alirocumab with placebo in 18 924 patients with recent acute coronary syndrome and elevated atherogenic lipoproteins despite optimized statin therapy. Primary outcome was major adverse cardiovascular events (MACE; coronary heart disease death, nonfatal myocardial infarction, fatal/nonfatal ischemic stroke, hospitalization for unstable angina). Associations between baseline apoB or apoB at 4 months and MACE were assessed in adjusted Cox proportional hazards and propensity score–matched models. Results: Median follow-up was 2.8 years. In proportional hazards analysis in the placebo group, MACE incidence increased across increasing baseline apoB strata (3.2 [95% CI, 2.9–3.6], 4.0 [95% CI, 3.6–4.5], and 5.5 [95% CI, 5.0–6.1] events per 100 patient-years in strata 35–<50, and ≤35 mg/dL, respectively). Compared with propensity score–matched patients from the placebo group, treatment hazard ratios for alirocumab also decreased monotonically across achieved apoB strata. Achieved apoB was predictive of MACE after adjustment for achieved low-density lipoprotein cholesterol or non–high-density lipoprotein cholesterol but not vice versa. Conclusions: In patients with recent acute coronary syndrome and elevated atherogenic lipoproteins, MACE increased across baseline apoB strata. Alirocumab reduced MACE across all strata of baseline apoB, with larger absolute reductions in patients with higher baseline levels. Lower achieved apoB was associated with lower risk of MACE, even after accounting for achieved low-density lipoprotein cholesterol or non–high-density lipoprotein cholesterol, indicating that apoB provides incremental information. Achievement of apoB levels as low as ≤35 mg/dL may reduce lipoprotein-attributable residual risk after acute coronary syndrome. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT01663402.gov; Unique identifier: NCT01663402.URL: https://www