*Review* **Semiconducting Polymer Dots for Point-of-Care Biosensing and In Vivo Bioimaging: A Concise Review**

**Sile Deng, Lingfeng Li, Jiaxi Zhang, Yongjun Wang, Zhongchao Huang \* and Haobin Chen \***

Department of Biomedical Engineering, School of Basic Medical Sciences, Central South University, Changsha 410013, China

**\*** Correspondence: bme\_hzc@csu.edu.cn (Z.H.); chenhb@csu.edu.cn (H.C.)

**Abstract:** In recent years, semiconducting polymer dots (Pdots) have attracted much attention due to their excellent photophysical properties and applicability, such as large absorption cross section, high brightness, tunable fluorescence emission, excellent photostability, good biocompatibility, facile modification and regulation. Therefore, Pdots have been widely used in various types of sensing and imaging in biological medicine. More importantly, the recent development of Pdots for point-of-care biosensing and in vivo imaging has emerged as a promising class of optical diagnostic technologies for clinical applications. In this review, we briefly outline strategies for the preparation and modification of Pdots and summarize the recent progress in the development of Pdots-based optical probes for analytical detection and biomedical imaging. Finally, challenges and future developments of Pdots for biomedical applications are given.

**Keywords:** polymer dots; biosensors; fluorescent probes; semiconducting polymers; molecular imaging

#### **1. Introduction**

Nanomedicine is the study of the application of nanoparticles in the field of biomedicine, and it has made progress in medical diagnosis and treatment, including biosensing, tissue engineering, medical imaging, cell tracking, drug transporting and cancer optical therapy [1–4]. Generally, biosensors can specifically detect analytes to provide physiological information in a fast and accurate way, and point-of-care testing has become a medical trend, as it greatly facilitates patient self-monitoring of health [5–8]. Apart from biosensing applications, biological imaging helps to visualize the internal structures or enables functional imaging for disease diagnosis, and multimodal imaging combines several imaging methods to integrate the respective signal containing different aspects of biological information for a more comprehensive diagnosis and accurate treatment [9,10]. With the development of materials and principles, biosensing and bioimaging technologies have received considerable attention due to their advantages of high resolution, real-time and non-invasiveness [11–13]. However, since the properties of the materials could exert influence on the sensitivity and accuracy of biosensing and optical applications, traditional small-molecule organic dyes suffer from inherent weakness such as short lifetime, poor photostability and low absorption, which limit further biomedical applications [14–16].

Nanomaterials with better properties and performance have been developed and widely used in the biological field [17–19]. Nanomaterials used to constitute biosensors have great properties and performance due to their unique nanoscale and easily modifiable characteristics, which benefit the energy transfer. On the other hand, nanomaterials for contrast agents contribute to better penetration depth and conversion efficiency, resulting in higher-quality imaging [20–22]. Typical luminescent nanomaterials include quantum dots (Qdots) [23], carbon dots [24], upconversion nanoparticles (UCNPs) [25], aggregationinduced emission (AIE) dots [26] and polymer dots (Pdots) [27]. In particular, Pdots have demonstrated the utilization of optical and biosensing applications in recent years, such as super-resolution imaging [28,29], fluorescence imaging [30,31] and disease-related marker

**Citation:** Deng, S.; Li, L.; Zhang, J.; Wang, Y.; Huang, Z.; Chen, H. Semiconducting Polymer Dots for Point-of-Care Biosensing and In Vivo Bioimaging: A Concise Review. *Biosensors* **2023**, *13*, 137. https:// doi.org/10.3390/bios13010137

Received: 16 December 2022 Revised: 11 January 2023 Accepted: 12 January 2023 Published: 14 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

detection [32,33]. Pdots are organic nanoparticles assembled from polymer chains with π-conjugated systems, and the nanoscale size endows Pdots with unique properties, which have attracted extensive attention. According to the definition given by Wu and Chiu [34], the Pdots, specifically considered as a part of semiconducting polymer nanoparticles (SPNs), are nanoparticles consisting of hydrophobic semiconducting polymers with a volume or weight fraction more than 50% and a diameter generally less than 50 nm, sometimes the particles size can be less than 30 nm. Pdots have shown characteristics of large absorption cross section, high brightness, good photostability, low toxicity and various forms of existence and modification, which are the basis of fluorescence probes for complex biological applications, typically for fluorescence-based biosensing and bioimaging [35–37].

This review focuses on the fundamental content and recent advances of Pdots in biosensing and bioimaging applications. The preparation and properties of Pdots are briefly introduced. Modification and functionalization are basic and crucial parts of practical applications, which are related to the attachment of functional groups to the surface of nanoparticles. Thus, several surface modification methods are also introduced. Many Pdots have been presented for in vitro biosensing applications and therapy applications, such as ion sensors [38], reactive oxygen species sensors [39], nucleic acid assays [40,41], enzymatic activity assays [42], photodynamic therapy [43], photothermal therapy [44], gene therapy [45] and chemotherapy [46], which are referred to in recent reviews [47–49]. Herein, we focus on the latest reported Pdots for point-of-care biosensing and in vivo imaging (Figure 1). Pdots point-of-care biosensors, applied to disease-related-metabolites assays, nicotinamide adenine dinucleotide (NAD) sensing, tumor markers quantification and cancer diagnostics, are detailed to demonstrate their great potential in biosensing and transducing techniques. Then, Pdots used as optical probes in bioimaging, such as fluorescence imaging, photoacoustic imaging (PAI), afterglow imaging, chemiluminescence imaging and multimodal imaging, are highlighted. The properties and biomedical applications of the Pdots summarized in this review are listed in Table 1. In the end, we share the challenges and perspective in this field.

**Figure 1.** Semiconducting polymer dots for biosensing and imaging.


**Table 1.** Pdots for biosensing and in vivo imaging.

N.A.: Not applicable.

#### **2. Semiconducting Polymer Dots**

This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, as well as the experimental conclusions that can be drawn.

#### *2.1. Methods of Preparation*

As different preparation methods generate the Pdots with different sizes and performance, it is critical to choose the corresponding method to obtain suitable size and properties according to various application scenarios. The main preparation methods include the direct polymerization method, miniemulsion method, nanoprecipitation method and self-assembly method. Direct polymerization, referring to the preparation of Pdots from low molecular weight monomers by chemical reactions, offers a wide range of options for size and structure since it also applies to the polymers that are insoluble in organic solvents [68]. Miniemulsion and nanoprecipitation methods dissolve conjugated polymers in organic solvents and interact with water [69,70]. The self-assembly method requires stirring of the solution to mix conjugated polymers and reagents for functionalization. In this part, nanoprecipitation and miniemulsion methods are mainly illustrated (Figure 2).

During the preparation process of the miniemulsion method, the conjugated polymers or monomers to be polymerized are dissolved in a water-immiscible organic solvent [71]. Under vigorous sonication or stirring, it forms microemulsion droplets with aqueous solutions containing surfactants. Finally, stable and uniformly-dispersed Pdots are obtained by removing the organic solvent. In particular, the surfactants are used to avoid aggregation of microemulsion droplets. The concentration of polymers and surfactants in the mixed solution can affect the size of Pdots.

**Figure 2.** The miniemulsion and nanoprecipitation methods of Pdots.

In the nanoprecipitation method, conjugated polymers and amphiphilic polymers are dissolved in a water-miscible organic solvent. Then, the mixed solution is rapidly injected into water under vigorous sonication, and the nanoprecipitation occurs during this process. Pdots with great water dispersibility are obtained by removing the organic solvent. The biggest difference between the miniemulsion and nanoprecipitation methods is the solvent. The nanoprecipitation method uses a water-miscible organic solvent such as tetrahydrofuran (THF), while the miniemulsion method uses a water-immiscible organic solvent such as chloroform. Typically, both methods use surfactants or amphiphilic polymers to increase the yield of nanoparticles. The size of the Pdots depends on the concentration of conjugated polymers in the organic solvents, which ranges from 5 to 50 nm, while the miniemulsion method often gives larger Pdots (larger than 40 nm). In addition, different kinds of amphiphilic polymers can realize different modifications for Pdots in the process of preparation. Liu's group fabricated uniform Pdots by a microfluidic-assisted nanoprecipitation process with a coaxial microfluidic glass capillary mixer [72]. Wu's group used the nanoprecipitation method to prepare functional Pdots with carboxyl groups on the surface for further bioconjugation [73]. Further, they combined photo-crosslinking technology to prepare Pdot-based nanocavities, nanoellipsoids, triangular nanorings and nanowires [74–76].
