Sheila N. Baker

Luminescent Carbon Nanodots: Emergent Nanolights

Similar to its popular older cousins the fullerene, the carbon nanotube, and graphene, the latest form of nanocarbon, the carbon nanodot, is inspiring intensive research efforts in its own right. These surface-passivated carbonaceous quantum dots, so-called C-dots, combine several favorable attributes of traditional semiconductor-based quantum dots (namely, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without incurring the burden of intrinsic toxicity or elemental scarcity and without the need for stringent, intricate, tedious, costly, or inefficient preparation steps. C-dots can be produced inexpensively and on a large scale (frequently using a one-step pathway and potentially from biomass waste-derived sources) by many approaches, ranging from simple candle burning to in situ dehydration reactions to laser ablation methods. In this Review, we summarize recent advances in the synthesis and characterization of C-dots. We also speculate on their future and discuss potential developments for their use in energy conversion/storage, bioimaging, drug delivery, sensors, diagnostics, and composites.

An analytical view of ionic liquids

Within the last decade, ionic liquids have come to the fore as environmentally-responsible designer solvents. But what are ionic liquids and what can they offer the analytical scientist? This article addresses these questions and chronicles recent progress made in the application of ionic liquids toward analytical problem-solving. While further progress is required before ionic liquids become mainstream analytical solvents, results to date commend their use in various modes of chemical analysis. It is our aim that the findings reported herein draw other researchers into this area and encourage the increased application of ionic liquids in this manner.

The large scale synthesis of pure imidazolium and pyrrolidinium ionic liquids

Ionic liquids are being employed in almost all areas of chemistry and materials, yet there are inherent issues which arise if the utmost care is not taken in the preparation and purification of these materials. They are not easily synthesized and purified using the existing methods. We describe a reliable method for producing large quantities of high quality ionic liquids. Additionally, we show that imidazoliums are not ‘special’ due to their ‘inherently fluorescent’ nature, that spectroscopically clean imidazoliums are attainable, and most classes of ionic liquids do exhibit fluorescent backgrounds when extreme care is not taken during their synthesis and purification.

Fluorescence studies of protein thermostability in ionic liquidsElectronic supplementary information (ESI) available: synthesis of [C4mpy][Tf2N]. See http://www.rsc.org/suppdata/cc/b4/b401304m/

Using the single tryptophan residue in the sweet protein monellin as a spectroscopic handle, we show the extreme thermodynamic stabilization offered by an ionic liquid; T(un) approximately 105 degrees C in [C4mpy][Tf2N] compared to 40 degrees C in bulk water.

Temperature-dependent microscopic solvent properties of ‘dry’ and ‘wet’ 1-butyl-3-methylimidazolium hexafluorophosphate: correlation with ET(30) and Kamlet–Taft polarity scales

As a result of heightened awareness of a wealth of potential in clean manufacturing processes, room temperature ionic liquids (RTILs) have been the target of increased investigation. As an integral part of the green chemistry movement, RTILs have found application in synthesis, catalysis, polymerization, industrial cleaning, liquid/liquid extraction, and separations. While some groundwork has been laid, the optimal utilization and tailoring of these solvents has been hobbled by an incomplete understanding of their solvent properties, particularly at the molecular level. In this work, we use solvatochromic measurements to determine ‘energy of transition’ ET(30) values and Kamlet–Taft solvent parameters (α, β, π*) for the relatively hydrophobic RTIL 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]+[PF6]−, as a function of temperature (10–70 °C) and water content (50 ppm or less water, ‘dry’ or 2% water (v/v), ‘wet’). The results of these experiments demonstrate that dry [bmim]+[PF6]− exhibits a hydrogen bond donor strength on the order of short chain alcohols with a linear temperature dependence. Dry and wet [bmim]+[PF6]− exhibit hydrogen bond acceptor abilities, which are weak functions of temperature, intermediate between that of water and acetonitrile. The π* parameter for wet and dry [bmim]+[PF6]− is higher than short chain alcohols, but lower than water or dimethylsulfoxide at ambient conditions, and it exhibits a strong linear temperature dependence. Finally, the addition of water to [bmim]+[PF6]− does not affect the β and π* values significantly.

Probing solute and solvent interactions within binary ionic liquid mixtures

In this communication, we explore solute–solvent and ion–ion interactions within binary ionic liquid mixtures. From the perspective of several solvatochromic reporter probes capable of a range of interactions, we have investigated the influence of mixture composition on the polarity, dielectric, hydrogen bonding interactions, and microfluidity within mixtures afforded by the ionic liquids 1-butyl-3-methylimidazolium bis(triflyl)imide, 1-ethyl-3-methylimidazolium bis(triflyl)imide, and 1-butyl-3-methylimidazolium hexafluorophosphate. Our results underscore the prevalence of hydrogen bond donor and competitive anion coordination effects, suggesting the possible use of such mixtures in the design and optimization of advanced media for separations or chemical processing.

A Simple Colorimetric Assay of Ionic Liquid Hydrolytic Stability

We describe a simple colorimetric method that can be used to assess the relative intrinsic hydrolytic stabilities of water-miscible and water-immiscible ionic liquids.

Protein-templated gold nanoclusters sequestered within sol–gel thin films for the selective and ratiometric luminescence recognition of Hg2+

Sequestration of bovine serum albumin (BSA)-stabilized gold nanoclusters (AuNCs@BSA) prepared using microwave assistance within sol-gel-derived mesoporous silica films permits the selective and highly sensitive quenchometric detection of aqueous Hg(2+) (limit of detection = 600 pM) with luminescence signal arising from oxidized BSA allowing for an analytically robust and reliable ratiometric detection. Overall, this work highlights a number of important advances, including the highest luminescence quantum yield reported to date for a protein-templated luminescent noble metal nanocluster (13%) made possible using a microwave-mediated synthesis followed by cold incubation. We also demonstrate the clear advantage of exploiting the luminescence signal arising from oxidized BSA as an internal reference to generate selectivity of response to Hg(2+). A careful Stern-Volmer quenching analysis reveals the persistence of two unique quenching sites for AuNCs@BSA entrapped within a sol-gel-derived glass, a minor population of which is unquenchable. Finally, based on these AuNCs@BSA nanosensors, we advise a path forward for paper-based indicator strip detection of heavy metals in aqueous streams, the implementation of which can be performed using the unaided eye, making it a meaningful approach for routine screening and in resource-limited situations.

Fluorescence correlation spectroscopic studies of diffusion within the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate.

Fluorescence correlation spectroscopy (FCS) measurements have been performed in order to determine the translational diffusion coefficients of three differently charged fluorescent probes (cationic: rhodamine 6G, neutral: 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4-H-pyran, DCM, and anionic: fluorescein) dissolved within the common room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6]. These experiments demonstrate that FCS is a rapid, sensitive, precise (typical RSD from 4-8%), and low-consumption screening tool for the determination of analyte mobility within microliter ionic liquid samples.

Fluorescent Probe Studies of Polarity and Solvation within Room Temperature Ionic Liquids: A Review

Ionic liquids display an array of useful and sometimes unconventional, solvent features and have attracted considerable interest in the field of green chemistry for the potential they hold to significantly reduce environmental emissions. Some of these points have a bearing on the chemical reactivity of these systems and have also generated interest in the physical and theoretical aspects of solvation in ionic liquids. This review presents an introduction to the field of ionic liquids, followed by discussion of investigations into the solvation properties of neat ionic liquids or mixed systems including ionic liquids as a major or minor component. The ionic liquid based multicomponent systems discussed are composed of other solvents, other ionic liquids, carbon dioxide, surfactants or surfactant solutions. Although we clearly focus on fluorescence spectroscopy as a tool to illuminate ionic liquid systems, the issues discussed herein are of general relevance to discussions of polarity and solvent effects in ionic liquids. Transient solvation measurements carried out by means of time-resolved fluorescence measurements are particularly powerful for their ability to parameterize the kinetics of the solvation process in ionic liquids and are discussed as well.