Abstract
Optical sensors based on single-walled carbon nanotubes (SWCNTs) demonstrate tradeoffs that limit their use in in vivo and in vitro environments. Sensor characteristics are primarily governed by the non-covalent wrapping used to suspend the hydrophobic SWCNTs in aqueous solutions, and we herein review the advantages and disadvantages of several of these different wrappings. Sensors based on surfactant wrappings can show enhanced quantum efficiency, high stability, scalability, and diminished selectivity. Conversely, sensors based on synthetic and bio-polymer wrappings tend to show lower quantum efficiency, stability, and scalability, while demonstrating improved selectivity. Major efforts have focused on optimizing sensors based on DNA wrappings, which have intermediate properties that can be improved through synthetic modifications. Although SWCNT sensors have, to date, been mainly engineered using empirical approaches, herein we highlight alternative techniques based on iterative screening that offer a more guided approach to tuning sensor properties. These more rational techniques can yield new combinations that incorporate the advantages of the diverse nanotube wrappings available to create high performance optical sensors.
Highlights
Optical sensors use light as a means of contactless detection for real-time sensing
This approach yielded a selective sensor for fibrinogen based on dipalmitoyl-phosphatidylethanolamine (DPPE)-PEG (5 kDa)-suspended single-walled carbon nanotubes (SWCNTs). This sensor was capable of detecting fibrinogen in a competitive binding assay in the presence of albumin, which can passivate the sensor by binding to non-specific binding sites (Bisker et al, 2016). This observation suggests that CoPhMoRe is more likely due to a combination of factors related to both the specific corona phase formed by the polymer-SWCNT complex and the unique elongated conformation of the fibrinogen protein, rather than sensing mechanisms based on aggregation, molecular weight, or protein hydrophobicity
Polymer wrappings in particular have served the dual purpose of both solubilizing SWCNTs and regulating the selectivity of SWCNTs toward specific analytes in biological media
Summary
Optical sensors use light as a means of contactless detection for real-time sensing. Distinct optical signals from a single device enables multimodal detection of several analytes simultaneously, a feature that is especially advantageous for remote in vivo biosensing applications. As described in several reviews (Boghossian et al, 2011; Liu et al, 2011; Kruss et al, 2013; Pan et al, 2017), single-walled carbon nanotubes (SWCNTs) are among the most promising fluorescence-based transducers for biosensing applications. Semiconducting forms of SWCNTs dispersed in aqueous solutions emit photoluminescence at near-infrared (near-IR) wavelengths (O’Connell et al, 2002) This emission lies within the optical transparency window for biological material (Boghossian et al, 2011) which, when coupled with the nanotube’s indefinite photostability and capabilities for single-molecule detection, makes SWCNTs attractive for in vivo continuous monitoring applications. The most common approach for noncovalently separating SWCNT bundles is liquid-phase exfoliation and stabilization (Coleman, 2009) This approach typically involves using forced dispersion (with sonication, for example) in the presence of wrappings, such as surfactants, synthetic polymers, oligonucleotides, and proteins that can stabilize the suspended SWCNTs (Figure 2). These examples highlight emerging methods to selectively engineer improved SWCNT-based optical sensors in complex environments
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