Mohammed and Omer Nanoscale Research Letters (2020) 15:182 https://doi.org/10.1186/s11671-020-03413-x NANO REVIEW Open Access Carbon Dots as New Generation Materials for Nanothermometer: Review Lazo Jazaa Mohammed and Khalid M. Omer* Abstract Highly sensitive non-contact mode temperature sensing is substantial for studying fundamental chemical reactions, biological processes, and applications in medical diagnostics. Nanoscale-based thermometers are guaranteeing non-invasive probes for sensitive and precise temperature sensing with subcellular resolution. Fluorescence-based temperature sensors have shown great capacity since they operate as “non-contact” mode and offer the dual functions of cellular imaging and sensing the temperature at the molecular level. Advancements in nanomaterials and nanotechnology have led to the development of novel sensors, such as nanothermometers (novel temperature-sensing materials with a high spatial resolution at the nanoscale). Such nanothermometers have been developed using different platforms such as fluorescent proteins, organic compounds, metal nanoparticles, rare- earth-doped nanoparticles, and semiconductor quantum dots. Carbon dots (CDs) have attracted interest in many research fields because of outstanding properties such as strong fluorescence, photobleaching resistance, chemical stability, low-cost precursors, low toxicity, and biocompatibility. Recent reports showed the thermal-sensing behavior of some CDs that make them an alternative to other nanomaterials-based thermometers. This kind of luminescent-based thermometer is promising for nanocavity temperature sensing and thermal mapping to grasp a better understanding of biological processes. With CDs still in its early stages as nanoscale-based material for thermal sensing, in this review, we provide a comprehensive understanding of this novel nanothermometer, methods of functionalization to enhance thermal sensitivity and resolution, and mechanism of the thermal sensing behavior. Keywords: Nanothermometer, Non-contact thermometer, Fluorescent thermometer, Carbon dots nanothermometer Introduction In history, the earliest estimation of temperature was Temperature is a fundamental thermodynamic variable feasibly built on sensation or observation. In ancient that has a remarkable influence on the biological and times, 200–10 BC, pneumatic experiments (air expansion chemical systems. On account of its wide range of by heat) assign as the oldest recognized references of ap- applications, almost in all fields of natural sciences, en- paratuses utilized for quantitative measurements of heat. gineering, agricultural, and medical sciences, precise Amid the most primitive writings related to heat ex- temperature determination is of great significance [1, 2]. panded air are credited to the works authored by Philo In medical applications, thermometry is used for early of Byzantium and Hero (or Heron) of Alexandria con- detection of various diseases, such as stroke, cancer, or cerning pneumatic experiments [3]. Later on, between inflammations, one of whose incipient symptoms is the the years 1592 to 1603, Galileo Galilei invented a ther- emergence of localized temperature peculiarities. moscope by setting experiments with the expansion of air by heat via the building of a simple apparatus using a tube encompassing air trapped above a column of water. * Correspondence: [email protected] Department of Chemistry, College of Science, University of Sulaimani, Qliasan Following Galileo, the Italian Santorio is accredited first Street, Sulaimani City, Kurdistan Region 46002,, Iraq for integrating this simple apparatus into medical © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Mohammed and Omer Nanoscale Research Letters (2020) 15:182 Page 2 of 21 examinations of fever. The first fully sealed liquid-in- for applications in which electromagnetic noise is strong, glass thermometer, alcohol-filled glass tube, was assem- sparks could be dangerous, the environment is destruc- bled by Ferdinand II in the year 1641. He was able to tive, or parts are promptly moving. Besides, traditional measure temperature unaided by barometric pressure, un- thermometers are not capable of measurements when like Galileo and Santorio’s open thermoscope. Fahrenheit’s spatial resolution lowers to the sub-micron scale, for in- practical work in thermometry emerged in 1706; he stance, in intracellular temperature variations and map- started with alcohol but subsequently became legendary ping the temperature of microcircuits and microfluidics for his mercury thermometers. Recognition for the centes- [9]. In the same way, engineering applications necessitate imal temperature scale has been provided to Anders Cel- advanced thermosensitive strategies for miniatured re- sius, who in 1742 projected a scale with zero at the gions and difficult environment [10]. Thus, for nano- temperature of water boiling and 100 at the temperature scale domains, one should think about other approaches of water freezing. Electronic experimentations performed and materials. in the nineteenth century when Thomas Johann Seebeck Novel non-contact thermometry can overcome the considered the concept of thermoelectricity. In a succes- aforementioned problems. For example, optical thermo- sion of experiments conducted between the years 1820 sensors (molecular thermometers) are a more recent and 1823, he verified the electrical potential in the generation of analytical tools that consists of molecular juncture-points of two different metals when there is a classes that use the measurement of emitted light to ex- heat difference between the joints. This was later known tract temperature [11, 12]. Fluorescent temperature- as the Seebeck Effect and it serves as the origin of the sensitive probes offer a promising area for thermometry thermocouple which is considered the most accurate in nano-system applications. The temperature informa- measurement of temperature [3–7]. The schematic time- tion can be extracted based on their fluorescence inten- line of the thermometer is shown in Fig. 1. sity, band shape, Stokes shift, or decay lifetime can relate Conventional thermometers can be categorized into: temperature if properly calibrated [13, 14]. Molecular thermometers have great potential in diag- 1. Liquid-filled glass thermometers based on the nosing diseased or carcinogenic cells, which have varying thermal expansion of materials physiological temperatures than ordinary cells. In the 2. Thermocouples based on the Seebeck effect medical applications, possibilities range from the 3. Optical sensors [8] temperature-induced control of gene expression [15] and cell metabolism [16] to the cell-selective penetration Furthermore, they can be classified as contact or non- and treatment of disease [17] and improving the heat contact thermometers. Contact mode, including classical dissipation from integrated heat sources [18]. Recently, liquid-filled glass, thermocouples, thermistors, and re- nanomaterials such as semiconductors [19], polymeric sistance temperature detectors (RTDs), all necessitate [10], and metallic nanoparticles [20] have been used as electrical cabling and a direct touch between the therm- thermal-sensor (nanothermometer) that showed sub- ometer and the substrate. This mode is not appropriate micron thermal resolution. Fig. 1 Timeline scheme for the evolution of thermometers Mohammed and Omer Nanoscale Research Letters (2020) 15:182 Page 3 of 21 Thermometers that can resolve sub-degree of acrylamide)) generally display poor photostability and pro- temperature over a wide array of temperatures that also nounced cross-sensitivity to oxygen, which is undesirable for can become integrated within living systems could offer live-cell work [8]. Additionally, there is a strong pH depend- an influential new tool in countless areas of biological, ence on the lifetime of the fluorophore, which makes it diffi- physical, and chemical research. Therefore, in this re- cult to use without the precise controlling of the pH of the view, we focus on a “new generation” or a “new class” of environment being surveyed [37]. nanothermometer that is based on carbon nanomaterials Another class of nanothermometers based on both pure (carbonaceous materials). To the best of our knowledge, and doped semiconductor nanocrystals has been reported, there is no reported review article on carbon dots as a with most prominent candidates such as CdSe, ZnS, InP, or nanothermometer. Recently, carbon dots (carbon PbSe [19, 38–40]. Semiconductor quantum dots (SQDs) are quantum dots, graphene quantum dots) along with their a candidate for nanoscale thermometers, considering they unique characteristics showed sensitive thermal