A facile and green approach to prepare carbon dots with pH-dependent fluorescence for patterning and bioimaging

As the new representative in the carbonaceous family, carbon dots (CDs) have gained remarkable research interests over the past decade. Herein, we report the facile preparation and thorough performance comparison of three types of carbon dots with the adoption of ubiquitous natural fruit juice as precursors and demonstrate their application in pH sensing, patterning and bioimaging. All the yielded CDs show interesting optical properties including evident single- or two-photon absorption and excitation-dependent photoluminescence along with the high fluorescent yield. A detailed study on the physical properties by EPR and Stokes shift analysis and structural composition analysis by XPS and Raman spectroscopy suggest that the fluorescence of CDs originates from the electron–hole recombination via the defect state. In addition, through the regulation of non-radiative recombination rate of CDs, all the obtained CDs could be applied as fluorescent sensing platforms toward the sensitive detection of the solution pH changes by the indication of CDs'' fluorescent yield and lifetime variation. Moreover, it was also proven that the resulting CDs could be employed as fluorescent inks for printing patterns in anti-counterfeit applications and as fluorescent probes for bioimaging of osteoblasts.

Carbon dots prepared with the adoption of ubiquitous natural fruit juices as precursors have good applications in pH sensing, patterning and bioimaging.     

Solvent-controlled synthesis of multicolor photoluminescent carbon dots for bioimaging

Multicolor fluorescent carbon dots (CDs) have potential applications in multichannel detection and multicolor imaging. In this study, multicolor fluorescent CDs were synthesized by changing the solvent type and adjusting the reactant ratio. The four prepared CDs emitted bright and stable blue (B-), green (G-), yellow (Y-), and red (R-) fluorescence under a single UV light (λ_ex = 365 nm). The photoluminescence (PL) emission wavelengths changed from 445 nm for B-CDs to 620 nm for R-CDs, and therefore covered the entire visible spectrum. The absolute quantum yields for the B-, G-, Y-, and R-CDs were 27.3%, 31.1%, 22.9%, and 8.8%, respectively. Characterization of the CDs showed that the differences among the optical features of the four prepared CDs arise from the differences among the surface states and nitrogen-derived structures in the carbon core. The four prepared CDs all showed low toxicity and steady PL, and therefore have potential applications in both in vitro and in vivo imaging.

The synthesis and bioimaging of multicolor carbon dots from citric acid and urea.     

Synthesis of green fluorescent carbon dots from carbon nano-onions and graphene oxide

In recent years, carbon dots (CDs) have triggered considerable interest due to their intriguing tunable photoluminescence properties. In this work, we report the synthesis of green-emitting CDs from two different carbon sources, namely carbon nano-onions and graphene oxide. We also investigate the effects of the two starting materials on the physico-chemical properties of the as-synthesised CDs. Our results show that both CDs exhibit remarkable emission properties and different fluorescence behaviour, which is attributed to the differences in size, surface defects, as well as the presence of different surface functional groups. Moreover, we propose an innovative, low-cost and time-saving method for the recovery of CDs from solution by acetone-mediated precipitation. We demonstrate that this methodology can rival the common dialysis-based purification approach; it shows excellent photostability, and the CD fluorescent properties are retained. Our work paves the way for the use of these particles for biomedical applications by exploiting their interesting fluorescent features as well as their oxygen-enriched surface for further functionalization strategies.

An easy and low-cost strategy for the synthesis of bright fluorescent CDs from CNOs and GO.     

N,S-co-doped carbon dots for rapid acid test paper and bioimaging

Fluorescent N,S-CDs with a quantum yield of 37.8% were synthesized via a one-pot solvothermal method. Detailed characterizations on physical, chemical and optical properties have been investigated. N,S-CDs demonstrated remarkably quenched and enhanced fluorescence in acidic and basic media. Direct qualitative analysis in pH sensor and cell imaging were preliminarily studied.

Fluorescent N,S-CDs with quantum yield of 37.8% were prepared via one-pot hydrothermal method for direct qualitative analysis in pH sensor and intracellular bioimaging.

Because of their excellent optical properties, low toxicity, photo-stability, high water solubility, easy synthesis and surface modification flexibility, fluorescent carbon dots (CDs) have attracted considerable attention for use in chemical and biological applications.^1–4 Recently, studies demonstrated CDs as one of the frontier fields in today''s fluorescent materials science and have become more and more important due to their high resolution and fast response.^5–7pH value, expressed as the negative logarithm of H⁺ concentration, plays an important role in environmental and biological processes like cell growth, diagenesis, and photosynthesis.^8–10 In recent years, fluorescent CDs-based nano-sensors have been designed for pH detection in both environmental monitoring and vivo/vitro bioanalysis. For these fluorescent pH sensors, fluorescence intensity, emission wavelength and fluorescence lifetime are measured to specifically respond to different pH values. In our previous report, we prepared CDs from acetic acid using a microwave method and the CDs showed pH-sensitive FL feature.¹¹ Liu et al. reported a green anhydrous approach for the preparation of hydrophilic CDs with pH-sensitive property between 3 and 13.¹² Zhang et al. prepared a red/orange dual-emissive CDs for pH monitoring from 1.0 to 13.0.¹³ Yu et al. reported a facile method for the synthesis of CDs with green fluorescence from phthalic acid and triethylenediamine hexahydrate. CDs was served as a pH nano-sensor.¹⁴ Compared to traditional organic dyes and semiconductor quantum dots, CDs-based sensors are superior in photo stability and low toxicity.^12,13 Most reported pH-responsive CDs exhibit a gradual change in fluorescence intensity by altering the pH value of the solution such that the pH can be detected. However, the fluorescence intensity often does not change much from one pH value to the neighbouring pH value. The testing process still needs to rely on fluorescence spectrometry for qualitative or quantitative analysis. Lack of visualization capability for directly performing qualitative analysis of the real sample under simple UV light by the naked eye is an obvious disadvantage.Herein, we described a facile and efficient hydrothermal fabrication pathway for N,S co-doped CDs (abbreviated as N,S-CDs) with high quantum yield (∼37.8%). The as-prepared N,S-CDs showed remarkably quenched and enhanced fluorescence in acidic and basic media. Their use in pH sensor and cell imaging were preliminarily studied.N,S-co-doped CDs (N,S-CDs) were synthesized by hydrothermal treatment of citric acid and N-methyl thiourea at 180 °C for 7 h and purified by filter and dialysis (Scheme 1). The reaction yielded a yellow-brown solution with strong blue emissions. Full synthetic details and conditions optimization (starting material ratio, heating temperature and reaction time) have been listed in the ESI (Fig. S1^†). Under the same optimal synthesis conditions, the reaction phenomena were consistent with various batches. The corresponding product yield could reach 69.1%.Open in a separate windowScheme 1The schematic of N,S-CDs preparation by hydrothermal pathway.The X-ray diffraction (XRD) pattern of the N,S-CDs displays a broad diffraction peak at around 2θ = 24.8° due to the interlayer spacing of 0.34 nm, while the weak peak at nearly 2θ = 41° represents the 0.21 nm interlayer spacing. The result is similar to that reported in other studies (Fig. S2^†).^15,16 X-ray photoelectron spectroscopy (XPS) was conducted to confirm the chemical composition and surface state of N,S-CDs. Full scan XPS spectrum exhibits four typical peaks of C_1s, N_1s, O_1s and S_2p at 168, 285, 400 and 531 eV with atomic contents of 53.5%, 13.1%, 25.8% and 7.6%, respectively (Fig. S3^†). The high-resolution spectra of C_1s, N_1s, O_1s and S_2p are shown in Fig S4.^† C_1s spectrum shows three carbon states of C O/C N/C S, C–C and C–O/C–S. Nitrogen exists in three forms of N–C₃, N–H and C–N–C. The O_1s spectrum can be resolved into two peaks attributed to C O and C–O/C–N relevant groups. S_2p spectrum can be ascribed to the 2p_3/2 and 2p_1/2 positions of the –C–S and –C–SO_X–. Therefore, the as-prepared N,S-CDs are rich in N and S heteroatoms.The fluorescence spectra of N,S-CDs solutions (pH = 7.0) are shown in Fig. S5.^† Increasing the excitation from 300 nm to 410 nm, the maximum emissions were located at 440 nm without shift, which demonstrated emission irrespective properties of the excitation wavelengths. However, CDs prepared from citric acid were observed as multi-color emissions (Fig. S6^†). It is generally agreed that doped N and S may modify the surface states and lead to non-radiative transitions between the carbogenic core and surface state.^17,18 Using quinine sulfate in H₂SO₄ (0.1 mol L⁻¹) as the standard, the quantum yield of N,S-CDs was measured up to be 37.8%, which is 6.5 times higher than that of CDs derived from citric acid (5.8%). According to previous reports, doped N and S could introduce more passivated surface defects and enhance the fluorescent properties.^19,20 Long time storage, continuous UV exposure and ionic strength show little effects on the FL intensity of the N,S-CDs, suggesting its good time-, photo- and ionic-stability (Fig. S7^†).In the process of research, we found that the FL intensity of N,S-CDs possessed interesting pH response property. As shown in Fig. 1a and b, the N,S-CDs show strong emissions at 440 nm when excited at 350 nm. Tuning the pH value to the acidic region, the intensity suddenly reduces and emits almost no fluorescence. When the pH value is higher than 7.0, the intensity increased nearly doubled. After changing the pH value 7 times from 5.0 to 9.0 and then back to 5.0, the intensity does not change much from the original values under the same pH conditions, which indicates good pH reversible performance and response ability of N,S-CDs (Fig. 1c). Subsequently, the fluorescence intensity response to some relevant interfering species has been investigated. According to the results in Fig S8,^† most metal ions and small bioactive molecules show little effect on the fluorescence, thus revealing good selectivity for pH monitoring based on the N,S-CDs.Open in a separate windowFig. 1(a) Fluorescence spectra (excited at 350 nm) of N,S-CDs in aqueous solution of different pH from 1 to 14; (b) change of normalized intensity with varying pH from 1 to 14; (c) the reversible pH-response of the FL behavior between pH 5.0 and 9.0 (excited at 350 nm); (d) optical photograph using the pH test paper prepared by N,S-CDs for the pH monitoring (left to right: pH = 5, 7, 9, 0.01 mol L⁻¹ of HAc solution and real lake water, UV beam: 365 nm).To collect insight on the size, morphology, surface chemistry and fluorescence change mechanism, N,S-CDs were characterized by TEM, UV and FTIR. Diluted N,S-CDs solutions (pH = 5, 7, 9) were dropped on carbon-coated copper grids for TEM measurement. The N,S-CDs solution of pH 5 showed a mono-dispersed spherical morphology with narrow size distribution between 1.5 and 4.0 nm (Fig. S9a^†). When the pH value increased to 7, the particles began aggregation and formed a network-like structure (Fig. S9b^†). In an alkaline environment (pH = 9), N,S-CDs are completely clustered together (Fig. S9c^†). From the high-resolution TEM micrographs of a single dot, well-resolved lattice fringes can be observed with a typical lattice spacing of 0.21 nm, which can be ascribed to the (0 0 2) in-plane lattice of graphene (Fig. S9d^†).¹⁹ FTIR spectra shown in Fig. S10^† demonstrate that some functional bonds of N,S-CDs in different pH environments such as C O (1700 cm⁻¹), O–H (3450 cm⁻¹), C–O (1250 cm⁻¹), C–S (1129 cm⁻¹) and C N/C S (1397 cm⁻¹). In acidic condition, the typical peaks of N–H band at 3130 cm⁻¹ and 1023 cm⁻¹ are strong. With the increase in pH value, the intensities of N–H vibrations reduced, while new peaks at 2064 cm⁻¹ and 1201 cm⁻¹ emerged, which could be ascribed to the conjugated double bonds and C–N groups. The difference of FTIR spectra in various pH conditions may have resulted from the protonation–deprotonation of surface groups and solvent effects.¹³ In acidic solution, the surface functional groups of N,S-CDs are protonated and positively charged. Additional electrophilic groups hinder the N,S-CDs from growing further. In neutral alkaline conditions, strong interactions between surface groups are enhanced because of polar–polar forces, van der Waal forces, H-bonding effects and electrostatic attraction, thereby increasing the conformational rigidity and affecting the morphology of N,S-CDs. The UV-vis absorption spectra of N,S-CDs with different pH values display strong absorption at high-energy region and a noticeable absorption band in a range of 380–450 nm, which is usually attributed to the π–π* transitions of C C core and n–π* transitions of surface C O/C N groups (Fig. S11^†).²¹ Increasing the pH value from 6 to 8, the n–π* absorption band red shifts from 324 to 335 nm. At pH = 8, new absorption band appears at around 600 nm, mainly assigned to the n–π* transitions of conjugated C S groups.¹³ The fluorescence emissions of the as-prepared N,S-CDs was primarily generated by the n–π* transitions of the surface aromatic system containing C O/C N bands. A small shift of the absorption peak may be attributed to the change of surface functional groups. Combined with the above analysis, aggregation of the N,S-CDs leads to the n–π* absorption and FL intensity increasing based on the aggregation induced enhancement effect.^22–24Based on the good pH responsiveness and excellent photostability of the N,S-CDs, fluorescent test paper for visual sensing of pH have been constructed. Briefly, the filter paper was soaked into the N,S-CDs solutions for 30 s, air-dried for 10 min, and then cut into small pieces for pH detection. The test paper exhibited strong blue emission under a UV lamp of 365 nm. Solutions with different pH (pH = 5, 7, 9) were injected into a pen for painting patterns on the test paper (Fig. 1d). It can be seen by directly observing the color under UV light. Acid solution strongly quenched the fluorescence, and alkaline solution could enhance the emission. The quenched color by acid can be clearly distinguished with the naked eyes; therefore, the test paper could be used for direct acid recognition. Diluted acetic acid and real lake water samples were studied based on the prepared test papers with clear color reaction.Intracellular pH (always around 7.4) is related to many physiological activities including cell growth and functions, which is of enormous importance in the fields of bio-medicine and molecular science.⁹ In view of good fluorescence properties and excellent physiological dispersion of the N,S-CDs, their fluorescence imaging in vitro has been studied. Firstly, the cytotoxicity to living cells has been investigated using HeLa cells as a model system. As shown in Fig. S12,^† the toxicity of the as-prepared N,S-CDs towards HeLa cells can be negligible even after 24 h. The cell viability is still higher than 80% at a concentration of about 600 μg mL⁻¹, which implies the biocompatibility of the N,S-CDs. To confirm the effectiveness of N,S-CDs in direct intracellular pH recognition, HeLa cells were co-cultured with N,S-CDs (100 μg mL⁻¹) at 37 °C for 4 h. Then, the cultured cells were treated in HEPES buffer with different pH (5.0, 7.0 and 9.0) and subjected to the confocal fluorescence imaging. As illustrated in Fig. 2, bright blue fluorescence signal of the HeLa cells could be observed after being incubated in HEPES buffer with pH = 7.0. When the pH is adjusted to 5.0, the images were quenched to nearly colorless. Moreover, when the pH increased to 9.0, the fluorescence was significantly increased. These observations illustrated a close correlation of fluorescence images intensity with the pH value in culture medium, which demonstrates that the N,S-CDs have potential applications in biological imaging and other associated biomedical applications.Open in a separate windowFig. 2Confocal fluorescence microscopy images of HeLa cells incubated with N,S-CDs (100 μg mL⁻¹) at 37 °C for 4 h in HEPES buffer with different pH of 5.0 (a), 7.0 (b) and 9.0 (c).     

Novel synthesis of fibronectin derived photoluminescent carbon dots for bioimaging applications

Fibronectin (FN) derived from human plasma has been used for the first time as the carbon precursor in the top-down, microwave-assisted hydrothermal synthesis of nitrogen doped carbon dots (CDs). FN is a large glycoprotein primarily known for its roles in cell adhesion and cell growth. Due to these properties FN can be over expressed in the extracellular matrix (ECM) of some cancers allowing FN to be used as an indicator for the detection of cancerous cells over non-cancerous cells. These FN derived CDs display violet photoluminescence with UV excitation and appear to possess similar functional groups on their surface to their carbon precursor (–COOH and –NH₂). This is believed to be due to the self-passivation of the CDs'' nitrogen-containing surface functional groups during the heating process. These CDs were then used to stain MCF-7 and MDA-231 breast cancer cells and were observed to interact primarily with the cell membrane rather than intercalating into the cell like the many other types of CDs. This led to the hypothesis that the CDs are selectively binding to the FN overexpressed within the cancer cells'' ECM via amide linkages. This is in agreement with the EDX and FTIR spectra of the FN CDs which indicate the presence of –COOH and nitrogen containing surface groups like –NH₃. The inherent selectivity of the CDs combined with their ability to photoluminesce enables their use as a fluorophore for bioimaging applications.

Fibronectin (FN) derived from human plasma has been used for the first time as the carbon precursor in the top-down, microwave-assisted hydrothermal synthesis of nitrogen doped carbon dots (CDs).     

Correction: Solvent-controlled synthesis of multicolor photoluminescent carbon dots for bioimaging

Correction for ‘Solvent-controlled synthesis of multicolor photoluminescent carbon dots for bioimaging’ by Yang Yan et al., RSC Adv., 2019, 9, 24057–24065.

The authors regret that the contributions of the authors were not correctly indicated in the original article. Yang Yan should be designated as the sole first author, and therefore the footnote stating “The two authors contributed equally” was in error. The correct author list is as shown above.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.     

Red,green, and blue light-emitting carbon dots prepared from o-phenylenediamine

Carbon dots (CDs), as the most important type of carbon-based material, have been widely used in many fields because of their excellent properties. In particular, multicolor fluorescent CDs with high photoluminescence quantum yield are the focus of active research. Herein, red, green and blue CDs (RGB CDs) were successfully synthesized by a solvothermal method from o-phenylenediamine under different reaction conditions. The RGB-CDs have stable optical properties and significant photoluminescence characteristics. Structural and elemental analyses propose a conjugated structure and the surface state of the CDs as the main causes for the different color emission of RGB-CDs. In addition, a white fluorescent CD solution was prepared by mixing these multicolor fluorescent CDs in appropriate proportions.

Red, green and blue CDs (RGB CDs) were successfully synthesized by a solvothermal method from o-phenylenediamine under different reaction conditions.     

Fluorescent sensing platform based on green luminescence carbon dots and AuNPs for clenbuterol detection in pork liver

In this paper, water-soluble green fluorescent carbon dots (G-CDs) were prepared using p-phenylenediamine and glutathione (GSH) as the precursors. The G-CDs exhibit excellent optical properties, and the maximum emission wavelength is located at 522 nm (under 410 nm excitation), which greatly overlaps with the absorption spectrum of AuNPs. Consequently, an effective “off–on” fluorescent sensing platform involved in G-CDs and AuNPs for detection of clenbuterol (CLB) was constructed. The fluorescence of G-CDs was strongly quenched by AuNPs due to the inner filter effect (IFE). As CLB was introduced, the quenched fluorescence intensity was recovered due to the specific interaction between the AuNPs and CLB. The recovered fluorescence intensity is linear to CLB concentration in the range of 13–270 ng mL⁻¹ with a low detection limit of 3.75 ng mL⁻¹. The prepared sensor has been successfully applied for CLB detection in pork liver and could be utilized in food analysis.

Carbon dots (G-CDs) with bright green fluorescence are synthesized by hydrothermal treatment of p-phenylenediamine and glutathione. Employing the G-CDs and AuNPs as sensing platform, a simple fluorescence sensor to detect clenbuterol was established.     

Dansyl-modified carbon dots with dual-emission for pH sensing,Fe3+ ion detection and fluorescent ink

In this work, a multifunctional ratiometric fluorescence (FL) nanohybrid (CSCDs@DC) was synthesized from chitosan based carbon dots (CSCDs) and dansyl chloride (DC) at room temperature. The CSCDs@DC revealed strong FL intensity, great stability and excellent anti-photobleaching properties. Herein, CSCDs@DC was responsive to pH value in the range of 1.5–4.0 and exhibited color-switchable FL properties between acidic and alkaline environments. In addition, CSCDs@DC showed good selectivity and sensitivity towards Fe³⁺ ions. A good linear relationship for the Fe³⁺ ion detection was obtained in the range from 0 μM to 100 μM, with a detection limit of 1.23 μM. What''s more, CSCDs@DC can be used as a fluorescent ink. It expressed superior optical properties after 3 months of storage or continuous exposure to UV light for 24 h. This study suggested that CSCDs@DC had potential in the detection of pH and metal ions, as well as showing promising application in the anti-counterfeiting field.

This work provided a new strategy for developing a multifunctional fluorescence platform which had potential in the detection of pH and metal ions, as well as showing promising application in the anti-counterfeiting field.     

Brewery spent grain derived carbon dots for metal sensing

This article presents a proof-of-concept to recycle microbrewery waste as a carbon source for synthesizing carbon dots (CDs). A simple method has been developed to synthesize water-soluble CDs based on microwave irradiation of brewery spent grain. The structures and optical properties of the CDs were characterized by ultraviolet-visible (UV-Vis) spectroscopy, photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. The effects of reaction time, temperature and pH on the properties of carbon dots were studied. These CDs were found to be spherical with an average diameter of 5.3 nm, N-doped, containing many functional groups (hydroxyl, ethers, esters, carboxyl and amino groups), and to exhibit good photoluminescence with a fluorescent quantum yield of 14%. Finally, the interaction between carbon dots and metal ions was investigated towards developing CDs as a sensing technology for water treatment, food quality and safety detection.

This article presents a proof-of-concept to recycle microbrewery waste as a carbon source for synthesizing carbon dots (CDs).     

A simple and green synthesis of carbon quantum dots from coke for white light-emitting devices

Coke is a by-product of coal. This paper reports a simple and green chemical oxidation method for carbon quantum dots (CQDs) from coke for use in novel applications. The CQDs emit blue fluorescence and have a fluorescence quantum yield of 9.2% and blue-green-red spectral composition of 48%. A light-emitting diode (LED) was fabricated by combining the CQDs as a white-light converter with an ultraviolet chip. The Commission Internationale de L''Eclairage chromaticity coordinate (0.31, 0.35) and correlated color temperature (5125 K) of the LED are located in a cool white light zone, suggesting that they have superior potential application in lighting devices.

CQDs are prepared from coke. The coke-based CQDs as a converter are applied to the white light illumination field.     

N-doped oxidized carbon dots for methanol sensing in alcoholic beverages

Methanol (MeOH) adulteration in alcoholic beverages resulting in irreparable health damage demands highly sensitive and cost-effective sensors for its quantification. As carbon dots are emerging as new biocompatible and sustainable light-emitting detectors, this work demonstrates the hydrothermally prepared nitrogen-doped oxidized carbon dots (NOCDs) as on-off fluorescent nanoprobes to detect MeOH traces in water and alcoholic beverages. The presence of 1% of MeOH in distilled water is found to decrease the NOCD fluorescent emission intensity by more than 90% whereas up to 70% ethanol (EtOH) content changes the signal to within 20% of its initial value. HR-TEM analysis reveals the agglomeration of the nanoprobes suspended in MeOH. Due to their selectivity towards MeOH, the fluorescent nanoprobes were successfully tested using a few MeOH spiked branded and unbranded Mexican alcoholic beverages. Varying degrees of signal quenching is observed from the fluorescent nanoprobes dispersed in different pristine beverages with a detection limit of less than 0.11 v%. Herein, we establish a new perspective towards economically viable non-toxic fluorescent probes as a potential alternative for the detection of MeOH in alcoholic beverages.

Herein, we establish a new perspective towards economically viable non-toxic fluorescent probes as a potential substitute of expensive alternative for the detection of MeOH in alcoholic beverages.     

The synthesis of green fluorescent carbon dots for warm white LEDs

Green fluorescent carbon dots (CDs) were synthesized with pyrogallic acid as carbon source by solvothermal method in N,N-dimethylformamide (DMF). During the formation of the CDs, DMF not only serves as solvent for reaction, but also as nitrogen source to participate in the reaction. At the same time, it promotes the formation of large conjugated sp²-domain in CDs. The prepared CDs have an average size of 11.9 nm, excitation-independent emission centered at 520 nm and fluorescence quantum yield of 16.8%. For practical applications, warm white light-emitting diodes were fabricated by combining the CDs/N-[3-(trimethoxysilyl)propyl]ethylenediamine (KH-792) mixture with UV chip, which emitted warm white light with color coordinates of (0.39, 0.47) and a correlated color temperature of 4323 K suitable for indoor lighting.

Green emissive carbon dots synthesized by solvothermal method is used for warm white light-emitting devices.     

pH and redox dual-sensitive drug delivery system constructed based on fluorescent carbon dots

Herein, a pH and redox dual-responsive drug delivery system (CDs–Pt(iv)–PEG) was developed based on fluorescence carbon dots (CDs). In this system, cisplatin(iv) prodrug (Pt(iv)) was selected as a model drug to reduce toxic side effects. The aldehyde-functionalized monomethoxy polyethylene glycol (mPEG-CHO) was conjugated to CDs–Pt(iv) to form pH sensitive benzoic imine bond. Owing to the slightly acidic tumor extracellular microenvironment (pH 6.8), the benzoic imine bond was then hydrolyzed, leading to charge reversal and decrease in the hydration radius of the drug-carrying, which facilitated in vivo circulation and tumor targeting. Notably, the cytotoxicity of the drug delivery system on cancer cells was comparable to that of cisplatin, while the side effects on normal cells were significantly reduced. In addition, the system realized recognition of cancer cells by the high-contrast fluorescent imaging. In conclusion, the CDs–Pt(iv)–PEG system provided a promising potential for effective delivery of anticancer drugs and cancer cells screening.

The system is pH-responsive and redox-controlled release. And the charge reversal and size transitions of the system can enhance the targeted ability. Moreover, the system can recognize the cancer cells by the fluorescence imaging.     

A simple preparation method of carbon dots by weak power bathroom lamp irradiation and their application for nimesulide detection and bioimaging

In this work, a novel strategy for synthesizing carbon dots (CDs) with a quantum yield of approximately 15.36% has been established by employing a bathroom lamp as a light source. Compared with other current protocols, the method described here displayed various advantages such as environmentally friendly manipulations and low power and cost. Subsequently, we applied the CDs as a fluorescence probe for the detection of nimesulide (Nim) firstly under the optimal conditions. A linear relationship between ln(F₀/F) and the concentration of Nim was obtained in the range from 0.5 μM to 75 μM with a detection limit of 100 nM. In addition, the as-prepared CDs showed excellent biocompatibility and were applied for cell imaging, which presented great potential applications in cell imaging.

This work reported the simple preparation method of carbon dots using weak power bathroom lamp irradiation, and explored their potential application in cell imaging and as a fluorescent sensor for the determination of nimesulide.     

A glassware-free combinatorial synthesis of green quantum dots using bubble wrap

Here, we describe the use of commercially-available bubble wrap as the basis for the simple, cheap combinatorial exploration of the synthesis of brightly emitting core/shell quantum dots.

In this communication, we highlight the use of bubble wrap in the simple parallel synthesis of CuInS₂-based quantum dots with different optical properties, based on varying precursors concentrations.

New chemical compounds are discovered at an astonishing rate; in 2015, the Chemical Abstract Service recorded its 100 millionth compound. Despite this, the actual hardware associated with synthetic chemistry has evolved at a much slower rate. Most chemical glassware is based on borosilicate glass (developed in 1915), with quartz, actinic and PTFE-coated glass used for analysis and storage.¹ There have been few developments in glassware until recently, when the Cronin group reported 3D-printed acetoxy silicone reaction-ware that could be used for cluster synthesis, with the geometry of the reaction vessel dictating the actual reaction product.² Further advances include the use of 3D-printed polypropylene modules that were combined and used to synthesise a simple drug.³One of the key developments in how reactions are carried out was the emergence of combinatorial chemistry and parallel synthesis, providing the ability to implement numerous chemical reactions simultaneously. When applied to material science, the seminal studies in combinatorial materials chemistry explored the preparation of superconducting materials using vapour deposition masks, allowing a library density of 10 000 samples per square inch.⁴ The emergence of materials chemistry has expanded beyond simple deposition techniques, and there exists a requirement for parallel solution chemistries to be developed. The use of combinatorial chemistry in material and nanoparticle science is well-documented, with luminescent materials (in which we are primarily interested) representing only a small fraction of what can be explored.⁵ Undertaking such work can be, however, expensive, requiring a large initial outlay for robotics kit for example, which makes such chemistry prohibitive for some developing laboratories. Also, not all combinatorial experiments need a sample density in the thousands; for some, tens of outputs are sufficient to arrive at a suitable positive outcome. Similarly, solution-based material science cannot rely on vapour deposition techniques.An important nanomaterial developed during the last three decades are the quantum dots. A combinatorial approach to the synthesis of CdSe quantum dots has been reported although this method utilised several microreactors and is beyond most synthetic laboratories.⁶ Likewise, a high-throughput robotic system has been used to explore the reproducible synthesis of various luminescent nanoparticles, giving unrivalled control and optimisation of the reactions; again, the instruments used in such a study are equally unique.⁷Inspired by a report from the Whiteside group which outlined the use of bubble wrap to store liquids, culture bacteria and take optical and electrochemical measurements, we describe the use of commercial bubble wrap as a simple reaction vessel and demonstrate multiple simultaneous reactions as a proof-of-principle for its use in basic combinatorial-style chemistry.⁸ Bubble wrap, as highlighted by Bwambok et al., has a variety of positive attributes that makes it ideal for simple aqueous-based chemistry. Of particular note are the low cost, ease of manipulation and disposal and the density of bubbles (library density) of up to 5000 m⁻², making this system particularly attractive to resource-poor laboratories. We also note that bubble wrap has been used to culture cells, carry out optical trapping experiments, and the colorimetric sensing of glucose.^9–11In this report, we have initially chosen the synthesis of core/shell CuInS₂/ZnS quantum dots as an example of where multiple simultaneous, parallel reactions can optimise luminescent nanoparticle synthesis; specifically, the optimum core compositions and shell precursor concentrations to achieve the brightest materials. CuInS₂ quantum dots were chosen as they are quickly emerging as key materials for numerous applications as they do not contain heavy metals, yet exhibit excellent optical properties, notably towards the red end of the visible spectrum. Such ‘green’ quantum dots may represent the next generation of materials for use in solar and biological applications, although such ternary solid-state materials have a wide variety of phases and stoichiometries.¹² Also, for the most basic applications, the majority of quantum dots require an inorganic shell layer to resist oxidation and confine charge carriers. Shell deposition is not simple, and often requires numerous attempts to reach the optimum thickness (bright point).¹³ If a shell is too thin, then the charge carriers are not sufficiently confined; if the shell is too thick, then the layer can exhibit defects that reduce the emission intensity. There, therefore, exists a complicated array of variables (for example, core material composition and shell thickness) that requires optimisation for core/shell quantum dot preparation.Whilst numerous reports exist on the various synthesis methods of the Cu/In/S quantum dot system¹² and the numerous resulting phases and materials, a key variable has not been explored in as much depth – that is, the optimum reaction conditions for luminescent core/shell materials. Determining the preferred core/shell structure in typical glassware reactions can be laborious and expensive, requiring numerous extensive, individual experiments and it would be beneficial if this could be decided simply and rapidly. In this work we report the exploration of several core precursor ratios and a shelling reaction in a few simple steps; the resulting brightest materials can be easily determined optically and then analysed and confirmed spectroscopically. We also describe how, after using bubble wrap to determine the key reaction parameters, we then carry out the entire reaction, from core synthesis to shell deposition in one single bubble, negating the need for standard glassware.Initially, whilst referring to previous reports,¹⁴ we prepared core CuInS₂ particles with a variety of Cu : In molar ratios (1 : 5, 1 : 10, 1 : 20, 1 : 40, 1 : 80), by pre-mixing the precursors for each ratio in separate vials, followed by injection of the reagents into a series of adjacent individual bubbles (ca. 5 mL volume, Fig. 1A for scale) on a single sheet, followed by heating the sheet in a water bath at 85 °C for 60 minutes. From these experiments, we determined by spectrometer that a range of materials with differing band edges and emissive properties resulted, confirming the variety of optical properties in this system. Materials prepared with a precursor ratio of 1 : 5 displayed absorption spectra with no excitonic features, whilst all other materials exhibited an excitonic feature at ca. 450 nm, although to varying degrees (ESI Fig. S1^†). The emission spectra showed a similar variation, with the 1 : 5 (Cu : In) ratio materials showing no evidence of emission, whilst the 1 : 10 ratio material clearly displayed two features, at ca. 550 nm and 650 nm (ESI Fig. S2^†). These emission features are strongly related to vacancies and defects, with the emission towards the red end of the visible spectrum reportedly associated with copper vacancies and donor–acceptor recombination.¹⁵ Features at ca. 535 nm have previously been observed by Macdonald et al. in CuInS₂ prepared in water, and again attributed to defects.¹⁶ By increasing the Cu : In ratio, the feature at ca. 650 nm decreased whilst the feature at ca. 550 nm became predominant. Despite emission being detected by spectrometer, none of the particles visibly fluoresced in the bubble wrap under 365 nm excitation, consistent with materials which exhibit a low emission quantum yield. The absorption spectra obtained using the reported method was similar to those obtained using an earlier synthetic method which was utilised for this work; the features exhibited no excitonic features, with onsets of absorption at ca. 600 nm. Emission spectra for similar reactions in glassware exhibited maxima between ca. 550 nm and 650 nm for core/shell materials, whilst emission spectra obtained for materials prepared in bubble wrap exhibited emission between ca. 550 nm and ca. 700 nm, extended into the red by a further 50 nm.¹⁴Open in a separate windowFig. 1(A) Photograph of quantum dot-filled bubbles with a £1 coin for size comparison; (B) photograph of bubbles filled with CuInS2/ZnS quantum dots (prepared with differing Cu : In ratios as shown) after synthesis; (C) photograph of materials in previous figure under 365 nm excitation; (D) normalised absorption spectra of CuInS₂/ZnS quantum dots prepared at different Cu : In ratios; (E) normalised emission spectra of CuInS₂/ZnS quantum dots prepared at different Cu : In ratios. This figure shows the range of optical properties available by varying precursor conditions in a simple bubble wrap synthesis.We then extended the synthesis to the preparation of a series of CuInS₂/ZnS quantum dots in bubble wrap by repeating the core synthesis using differing Cu : In ratios as described above. Following this, a set amount of ZnS shell precursors were added to each bubble and once resealed, the sheet was then reheated to 85 °C for a further 60 minutes. The resulting array of CuInS₂/ZnS quantum dots exhibited different colours (Fig. 1B), which were luminescent when illuminated at 365 nm (Fig. 1C), clearly emitting at different wavelengths. It was observed that the deposition of a ZnS shell on CuInS₂ core nanoparticles prepared by different precursor ratios produced quantum dots of varying band edge positions as suggested by the difference in colour of the samples, and different emissive colours and brightness. The materials were analysed spectroscopically (Fig. 1D and E) and the range of absorption and emission wavelengths were confirmed. The absorption band edges of the core/shell materials lost the sharp profile and all excitonic features yet remained in the same approximate spectral region (normalised spectra, Fig. 1C). The most surprising results were observed in the emission spectra, where the core/shell sample with Cu : In ratio of 1 : 5 exhibited an emission profile at ca. 700 nm (Fig. 1E), whereas the core material alone did not display any emission. Notably, all emission appeared composed of a single feature with similar full width at half the maximum (FWHM) of ca. 110 nm, with no evidence of the previous secondary feature at ca. 550 nm, in agreement with previous studies which attributed the high energy emission suppression to zinc passivation of donor defect sites.^15,16 Increasing the Cu : In ratio resulted in a gradual blue shift in the emission maxima, to a minimum wavelength of ca. 550 nm for core/shell samples prepared with a 1 : 80 core Cu : In precursor ratio. By eye, it was determined that CuInS₂/ZnS nanoparticles with core Cu : In precursor ratios of 1 : 10 and 1 : 20 resulted in the brightest materials, although both emitted at different wavelengths. It should be stated that despite significant work into the origin of emission from Cu/In/S nanomaterials, no clear opinion has been reached; although it is clear that radiative emission does not originate from a simple quantum confined band edge; rather from an as-yet unconfirmed defect state(s).Once we had ascertained that addition of a ZnS shell to core materials prepared with a 1 : 5 precursor ratio resulted in a luminescent structure (whereas the core material alone was non-luminescent, as determined visually and spectroscopically), we explored the potential for further enhancing the emission by varying the amount of ZnS precursors, potentially providing a thicker shell and hence exploring the opportunity to uncover a bright point. As can be seen from Fig. 2A–D, varying the amounts of shell precursor with extended heating had minimal effect on spectral position or emission brightness. Spectroscopic examination of the materials prepared in the bubbles displayed similar absorption edges (all exhibiting the same band edge position with a slight suggestion of an excitonic feature at ca. 500 nm as shown in Fig. 2C) whilst emission spectra appeared to reduce slightly in intensity with the addition of shell precursors (Fig. 2D) whilst maintaining the same spectral position, showing no evidence of exciton leakage into the shell (Fig. 2D, inset).Open in a separate windowFig. 2(A) Photograph of CuInS₂ quantum dots after various volumes (numbers shown in mL) of ZnS precursor had been added; (B) photograph of materials in previous figure under 365 nm excitation; (C) normalised absorption spectra of CuInS₂/ZnS quantum dots prepared using different amounts of ZnS precursor; (D) emission spectra of CuInS₂/ZnS quantum dots prepared using different amounts of ZnS precursor. Inset, normalised spectra showing spectral position. The data shows that addition of varying amounts of shell precursor had little effect on the optical properties of CuInS₂/ZnS.To ensure that the potential of bubble wrap synthesis extends beyond simple combinatorial-style synthetic experiments (where starting materials were injected from a precursor reservoir into numerous bubbles) and to mimic typical glassware-based reaction where precursors are sequentially injected, we prepared CuInS₂ quantum dot with a Cu : In ratio of 1 : 10, followed by addition of 0.2 mL ZnS precursor solution, entirely in a single bubble in a stand-alone experiment by sequentially injecting precursors and heating. In this case, a phosphate buffer solution of the metal precursors and capping agent (l-cysteine) were injected into a bubble then sealed. Following this, the sulfur precursor was injected and the bubble resealed, left for one hour, then heated in a water bath at 85 °C for one hour. Shell addition as described above, completed the reaction, the product of which was found to brightly luminesce when excited at 365 nm (Fig. 3A and B). Spectroscopic analysis of two identical reactions in adjacent bubbles confirmed almost identical optical band edges, whilst the emission profiles shifted position slightly (Fig. 3C and D). This confirms that pre-mixing precursors before addition to a bubble is unnecessary and that bubble wrap can be used as a stand-alone reaction vessel. Whilst the method can be easily adapted to aqueous-based quantum dot synthesis, there are obvious limitations. This method cannot, as yet, be used to make quantum dots at high temperatures in organic solvents, which is considered to be the most popular method. There are also no facilities to involve mixing, although this has not been an issue to date and we assume convective heating dominates solvent volumes as small as 5 mL, although this is clearly one factor that could impact which nanomaterial product is obtained; such issues have been previously highlighted as important in the high temperature synthesis of cobalt nanoparticles.¹⁷ Also, this method relies on the initial analysis of colours and intensities by eye, with no correction for mistaking optical density/colour or consideration for the eye''s response to green over red, for example. One should note however, that the ultimate use for these particles may be displays, which also rely on the eye''s inherent optical response. Likewise, optically-based assays have been developed that allow antigen detection at the femto-gram per milliliter level by the naked eye.¹⁸Open in a separate windowFig. 3(A) Photograph of two sets of CuInS₂/ZnS quantum dots prepared by a stand-alone method in two adjacent bubbles using the optimised conditions (Cu : In ratio of 1 : 10, followed by addition of 0.2 mL ZnS precursor solution); (B) photograph of CuInS₂/ZnS quantum dots used in previous figure excited at 365 nm; (C) normalised absorption spectra of the two samples shown in previous figures; (D) normalised emission spectra of the two samples shown in previous figures. This data shows that CuInS₂/ZnS quantum dots can be simply prepared using sequential precursor addition in a bubble, and that the method is reproduceable.In conclusion, we have demonstrated that bubble wrap can be used in a combinatorial style set of experiments to determine the optimum reaction conditions for brightly luminescent core/shell quantum dots. We also demonstrated that bubble wrap can be used as a stand-alone reaction vessel, withstanding numerous reagent injections and heating, which should be applicable to other simple chemical reactions.     

Label-free detection of creatinine using nitrogen-passivated fluorescent carbon dots

In the field of biochemistry and biosensing, the passivation of carbon dots using nitrogen dopants has attracted great attention, as this can control their photoluminescence (PL) properties and quantum yield. To date, in the fabrication of a sensing probe, the impact of the chemical composition of the passivating molecule remained unexplored. In this work, we chose a series of different nitrogen-rich precursors (such as urea, thiourea, cysteine, and glycine) and ascorbic acid to synthesize nitrogen-doped carbon dots (NCDs). A significant change in their surface states was obtained due to the evolution of variable contents of amino, pyridinic and pyrrolic nitrogen species, which is evident from X-ray photoelectron spectroscopy, and this leads to an increment in their PL quantum yields (PLQY ∼ 58%) and average lifetime values. Spectroscopic analysis revealed that a rise in the ratio of pyrrolic : amino groups on the surface of carbon dots cause a bathochromic shift and generate excitation-dependent properties of NCDs. Besides, these NCDs were used as fluorescence off–on sensing probes, where a PA-infested NCD solution was used to detect creatinine. Chiefly, fluorescence restoration was achieved due to the formation of Jaffe chromogen between creatinine and PA. However, all nitrogen-passivated carbon dot surfaces do not respond similarly towards creatinine and only non-amino-rich NCDs exhibit the maximum (50%) PL turn-on response. The PL turn-off–on methodology showed a satisfactory good linearity range between 0 and 150 μM with a detection limit of 0.021 nM for creatinine. Three input molecular logic gates were also designed based on the turn-off–on response of the NCDs@PA towards creatinine. Additionally, for analytical method validation, real-sample analysis was performed for creatinine, which showed good recoveries (93–102%) and verified that nitrogen passivation tailored the physicochemical properties and enhanced the sensing ability.

The role of passivation in CDs using different nitrogen precursors to evaluate its sensing proficiency towards creatinine quantification.     

One-step synthesized amphiphilic carbon dots for the super-resolution imaging of endoplasmic reticulum in live cells

Stimulated emission depletion (STED) microscopy provides a powerful tool for visualizing the ultrastructure and dynamics of subcellular organelles, however, the photobleaching of organelle trackers have limited the application of STED imaging in living cells. Here, we report photostable and amphiphilic carbon dots (Phe-CDs) with bright orange fluorescence via a simple one-pot hydrothermal treatment of o-phenylenediamine and phenylalanine. The obtained Phe-CDs not only had high brightness (quantum yield ∼18%) but also showed excellent photostability under ultraviolet irradiation. The CDs can quickly penetrate into cells within 2 min and are specific for intracellular ER. The further investigations by Phe-CDs revealed the reconstitution process of ER from loosely spaced tubes into a continuously dense network of tubules and sheets during cell division. Importantly, compared with the standard microscopy, STED super-resolution imaging allowed the tracking of the ER ultrastructure with a lateral resolution less than 100 nm and the pores within the ER network are clearly visible. Moreover, the three dimensional (3D) structure of ER was also successfully reconstructed from z-stack images due to the excellent photostability of Phe-CDs.

Amphiphilic carbon dots (Phe-CDs) were synthesized directly via one-step hydrothermal reaction for specific ER targeting without further modification. The Phe-CDs were photostable enough to allow STED super-resolution imaging of ER in live cells.     

Orange emissive carbon dots for colorimetric and fluorescent sensing of 2,4,6-trinitrophenol by fluorescence conversion

In this study, infrequent orange carbon nanodots (CNDs) were applied as a dual-readout probe for the effective colorimetric and fluorescent detection of 2,4,6-trinitrophenol (TNP). The orange fluorescence could be rapidly and selectively quenched by TNP, and the colorimetric response from the original pink color to blue could also be captured immediately by the naked eye. A limit of detection of 0.127 μM for TNP was estimated by the fluorescent method and 5 × 10⁻⁵ M by visualized detection. Interestingly, the fluorescence of the CNDs with TNP gradually transitioned from orange to green upon irradiation by a UV lamp, and the colorimetric response transitioned from pink to blue to colorless, which ensured effective multi-response detection of TNP. In addition, the CNDs exhibited bright fluorescence, excellent biocompatibility and low toxicity, making them high-quality fluorescent probes for cellular imaging.

We have described a colorimetric and fluorescent dual-readout probe with a strong and sensitive response towards TNP.     

Fluorescent probe based on N-doped carbon dots for the detection of intracellular pH and glutathione

Carbon dots (CDs) as fluorescent probes have been widely exploited to detect biomarkers, however, tedious surface modification of CDs is generally required to achieve a relatively good detection ability. Here, we synthesized N-doped carbon dots (N-CDs) from triethylenetetramine (TETA) and m-phenylenediamine (m-PD) using a one-step hydrothermal method. When the pH increases from 3 to 11, the fluorescence intensity of the N-CDs gradually decreases. Furthermore, it displays a linear response to the physiological pH range of 5–8. Au³⁺ is reduced by amino groups on the surface of N-CDs to generate gold nanoparticles (AuNPs), causing fluorescence quenching of the N-CDs. If glutathione (GSH) is then added, the fluorescence of the N-CDs is recovered. The fluorescence intensity of the N-CDs is linearly correlated with the GSH concentration in the range of 50–400 μM with a limit of detection (LOD) of 7.83 μM. The fluorescence probe was used to distinguish cancer cells from normal cells using pH and to evaluate intracellular GSH. This work expands the application of CDs in multicomponent detection and provides a facile fluorescent probe for the detection of intracellular pH and GSH.

N-doped carbon dots used as a fluorescence probe can distinguish cancer cells from normal cells by pH and evaluate intracellular GSH.