Quantum dots (QDs) have received great interest for diverse applications due to their distinct advantages, such as narrow and symmetric emission with tunable colors, broad and strong absorption, reasonable stability, and solution processibility. Doped QDs not only potentially retain almost all of the above advantages, but also avoid the self-quenching problem due to their substantial ensemble Stokes shift. Two obvious advantages of doped QDs, especially doped ZnS QDs, over typical CdSe@ZnS and CdTe QDs are longer dopant emission lifetime and potentially lower cytotoxicity. The lifetime of dopant emission from transition-metal ion or lanthanide ion-doped QDs is generally longer than that of the bandgap or defect-related emission of host, and that of biological background fluorescence, providing great opportunities to eliminate background fluorescence for biosensing and bioimaging. For bioimaging applications, fluorescent dopants may mitigate toxicity problems by producing visible or infrared emission in nanocrystals made from less-harmful elements than those currently used. In this review, recent advances in utilizing doped QDs for chemo/biosensing and bioimaging are discussed, and the synthetic routes and optical properties of doped QDs that make them excellent probes for various strategies in chemo/biosensing and bioimaging are highlighted. Moreover, perspectives on future exploration of doped QDs for chemo/biosensing and bioimaging are also given.

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Integrating various enzymes with nanomaterials provides various nanohybrids with new possibilities in biosensor applications. Furthermore, the enzymatic activity and stability are also improved due to the large surface area of nanomaterials. Here we report the conjugation of glucose oxidase (GOD) onto phosphorescent Mn-doped ZnS quantum dots (QDs) using 1-ethyl-3-(3-dimethylaminopropy)carbodiimide (EDC)/N-hydroxysuccinimide (NHS) as coupling reagents for glucose biosensing based on the effective quenching of the room temperature phosphorescence (RTP) of Mn-doped ZnS QDs by the H(2)O(2) generated from GOD-catalyzed oxidation of glucose. The obtained bioconjugate not only provided improved enzymatic performance with Michaelis-Menten constant of 0.70 mM but also favored biological applications because the phosphorescent detection mode avoided the interference from autofluorescence and scattering light from the biological matrix. In addition, the GOD-conjugated Mn-doped ZnS QDs showed better thermal stability in the temperature range of 20-80 degrees C. The GOD-Mn-doped ZnS QDs based RTP sensor for glucose gave a detection limit of 3 microM and two linear ranges from 10 microM to 0.1 mM and from 0.1 to 1 mM. The developed biosensor was successfully applied to the determination of glucose in real serum samples without the need for any complicated sample pretreatments.

Materials for photosensitized oxygen activation are extremely important for a suite of photodynamic applications in biomedical, analytical, and energy sectors. Carbon-based photosensitizers are attractive for their low cost and high stability, but most of them such as fullerene and graphene quantum dots suffer from low efficiency, and the rational design of carbon-based photosensitizers remains a challenge. Given the similar chemical origin of phosphorescence and photosensitization, we herein synthesized a series of nitrogen-doped carbon dots (C-dots) and confirmed that their photo-oxidation activity correlated with their phosphorescence quantum yields, providing a direction for the rational designing of such materials. Compared to other carbon nanomaterials and molecular photosensitizers, these C-dots have the highest activity, and they can finish oxidation reactions in a few seconds. The excellent photosensitized oxygen activation makes these water-soluble C-dots a promising oxidase-mimicking nanozyme for photodynamic antimicrobial chemotherapy and other applications.

Semiconductor quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over organic dyes as optical labels for chemo/bio-sensing. This review addresses the methods for metal ion detection with QDs, including photoluminescent, electrochemiluminescent, photoelectrochemical, and electrochemical approaches. The main mechanisms of direct interaction between QDs and metal ions which lead to photoluminescence being either off or on, are discussed in detail. These direct interactions provide great opportunities for developing simple yet effect metal ion probes. Different methods to design the chemically-modified QD hybrid structures through anchoring metal ion-specific groups onto the surface of QDs are summarized. Due to the spatial separation of the luminescence center and analyte recognition sites, these chemically-modified QDs offer greatly improved sensitivity and selectivity for metal ions. Several interesting applications of QD-based metal ion probes are presented, with specific emphasis on cellular probes, coding probes and sensing with logic gate operations.

Lab-on-a-nanoparticle: the triple-channel optical properties of Mn-doped ZnS quantum dots (fluorescence, phosphorescence, and light scattering) are explored to develop a multidimensional sensing device for the discrimination of proteins in a lab-on-a-nanoparticle approach.

Ratiometric fluorescent sensors, which can provide built-in self-calibration for correction of a variety of analyte-independent factors, have attracted particular attention for analytical sensing and optical imaging with the potential to provide a precise and quantitative analysis. A wide variety of ratiometric sensing probes using small fluorescent molecules have been developed. Compared with organic dyes, exploiting semiconductor quantum dots (QDs) in ratiometric fluorescence sensing is even more intriguing, owing to their unique optical and photophysical properties that offer significant advantages over organic dyes. In this review, the main photophysical mechanism for generating dual-emission from QDs for ratiometry is discussed and categorized in detail. Typically, dual-emission can be obtained either with energy transfer from QDs to dyes or with independent dual fluorophores of QDs and dye/QDs. The recent discovery of intrinsic dual-emission from Mn-doped QDs offers new opportunities for ratiometric sensing. Particularly, the signal transduction of QDs is not restricted to fluorescence, and electrochemiluminescence and photoelectrochemistry from QDs are also promising for sensing, which can be made ratiometric for correction of interferences typically encountered in electrochemistry. All these unique photophysical properties of QDs lead to a new avenue of ratiometry, and the recent progress in this area is addressed and summarized here. Several interesting applications of QD-based ratiometry are presented for the determination of metal ions, temperature, and biomolecules, with specific emphasis on the design principles and photophysical mechanisms of these probes.

Functionalization of a gold surface with DNA is often complicated by kinetic traps from unintended DNA base adsorption. Herein, we communicate that Br serves as a robust backfilling agent displacing selected DNA bases on gold. Traditional thiol backfillers are too strong, while even 300 mM Br is well tolerated. Conjugates prepared with Br hybridize 10-fold faster and resist DNA release with better colloidal stability yielding highly sensitive probes. From colorimetric and Raman assays, adsorption affinity ranks as F < T ≈ Cl < C < G ≈ Br < A < I, allowing Br to displace nonpoly-A sequences from gold. This well-controlled biointerface will impact biosensing, drug delivery, and directed assembly of nanomaterials.