Preparation method of red light carbon dots with long stokes shift

Abstract

This invention belongs to the field of fluorescent luminescent materials technology, and relates to a method for preparing red carbon dots with a long Stokes shift. The technical problem to be solved is that although existing red carbon dots have emission wavelengths exceeding 600 nm, their Stokes shifts are too small, resulting in defects such as fluorescence self-absorption, severe overlap between excitation and emission spectra, and easy triggering of fluorescence quenching during complexation with polymer carriers or biomolecules. The technical solution includes: mixing p-phenylenediamine and anhydrous ethanol at a mass-to-volume ratio of 0.01 g / mL to 0.02 g / mL, and ultrasonically treating to obtain a homogeneous mixture; placing the mixture in a high-pressure reactor and performing a solvothermal reaction at 180°C to 220°C for 4 to 8 hours; after the reaction, cooling to room temperature, collecting the reaction solution, concentrating under reduced pressure, dialyzing the concentrated solution in a dialysis bag with deionized water for purification, and finally freeze-drying to obtain the red carbon dots.

Description

Technical Field

[0001] This invention belongs to the field of fluorescent luminescent materials technology and provides a method for preparing red carbon dots with long Stokes shift. Background Technology

[0002] In recent years, carbon dots (CDs), as a novel zero-dimensional carbon nanomaterial, have shown great application potential in biomedical imaging, drug delivery tracking, and biosensing due to their excellent biocompatibility, low cytotoxicity, good photostability, and ease of surface functionalization. Compared to traditional semiconductor quantum dots containing heavy metals or organic fluorescent dyes that are prone to photobleaching, carbon dots are considered a new generation of safer and more ideal biofluorescent probes.

[0003] However, most reported carbon dots emit short-wavelength fluorescence when excited by ultraviolet or short-wavelength visible light (such as blue and green light). In practical biomedical applications, biological tissues (such as blood, fat, and skin) have strong absorption and scattering effects on short-wavelength light, resulting in extremely shallow light penetration depth. Simultaneously, endogenous substances within the organism produce strong autofluorescence background under short-wavelength excitation, severely interfering with the acquisition of target signals. Therefore, developing red carbon dots with emission wavelengths located in the optical window of biological tissues (the red to near-infrared region above 600 nm) is crucial for achieving high-contrast in vivo imaging of deep tissues.

[0004] Furthermore, in the development of red-light carbon dots, the Stokes shift (the difference between the excitation and emission wavelengths) is another core parameter determining their imaging quality. Existing red-light carbon dots generally suffer from a small Stokes shift. A small Stokes shift leads to severe fluorescence self-absorption, significantly reducing the material's actual fluorescence quantum yield. More critically, the excitation and emission spectra overlap, making it difficult to effectively separate the weak fluorescence signal from the excitation light scattering background in complex physiological environments, greatly reducing the signal-to-noise ratio. Moreover, when carbon dots are composited with other polymer carriers or biomolecules, a small Stokes shift easily triggers complex energy transfers, leading to fluorescence quenching. Summary of the Invention

[0005] The purpose of this invention is to solve the triple technical defects caused by the small Stokes shift (usually less than 100 nm) of existing red carbon dots (although the emission wavelength is above 600 nm):

[0006] The fluorescence self-absorption effect leads to a significant decrease in quantum yield;

[0007] The excitation and emission spectra overlap significantly, making it difficult to effectively separate the weak fluorescence signal from the physiological scattering background, resulting in a low signal-to-noise ratio.

[0008] Furthermore, it is prone to triggering non-radiative energy transfer and fluorescence quenching during complexation with polymer carriers or biomacromolecules, thus overcoming its performance bottleneck in high-contrast bioimaging applications in deep tissues.

[0009] To achieve the above objectives, the present invention employs the following technical means:

[0010] This invention provides a method for preparing red carbon dots with long Stokes shift, comprising the following steps:

[0011] Step (1): Mix p-phenylenediamine and anhydrous ethanol at a mass-to-volume ratio of 0.01-0.02 g / mL, and sonicate to obtain a homogeneous mixture;

[0012] Step (2): Place the homogeneous mixture in a high-pressure reactor and carry out a solvothermal reaction at 180-220℃ for 4-8 hours;

[0013] Step (3): After the reaction is completed, cool to room temperature, collect the reaction solution, concentrate the reaction solution under reduced pressure, place the concentrate in a dialysis bag and dialyze it in deionized water for purification, and finally freeze-dry to obtain red carbon dots.

[0014] Preferably, in step (2), the homogeneous mixture is placed in a high-pressure reactor and subjected to a solvothermal reaction at 200°C for 6 hours.

[0015] In the above scheme, the ultrasonic treatment time in step (1) is 30 minutes.

[0016] In the above scheme, the dialysis bag used in step (3) has a molecular weight cutoff of 500 Da, the dialysis time is 48 hours, and the dialysis solution is replaced every 8 hours.

[0017] In the above scheme, the mass ratio of the p-phenylenediamine precursor to the volume ratio of anhydrous ethanol in step (1) is 0.3 g: 30 mL.

[0018] The present invention also provides a red carbon dot, which is prepared by the preparation method described above.

[0019] The fluorescence emission wavelength of the aforementioned red carbon dots is 612 nm, and the Stokes shift is 144 nm.

[0020] Because the present invention employs the above-mentioned technical means, it has the following beneficial effects:

[0021] This invention uses phenylenediamine as a precursor and controls the reaction time and the type of catalyst to successfully obtain red carbon dots with emission wavelengths located in the optical window of biological tissue. This effectively distinguishes the emission characteristics of the carbon dots from the autofluorescence of biological tissue, solving the technical problems of extremely shallow light penetration depth and susceptibility to strong absorption and scattering interference from biological tissue in traditional fluorescent nanomaterials in biological in vivo imaging. It achieves the red-shifting of the emission wavelength of carbon dots to the optical window of biological tissue, thereby significantly improving the penetration ability of light in biological tissue and realizing the effect of high-contrast in vivo imaging of deep tissues.

[0022] This invention reconstructs the surface energy level structure of carbon dots by precisely controlling the conditions of the reaction system. This solves the technical problems of existing red light luminescent materials, such as small Stokes shift leading to severe fluorescence self-absorption (fluorescence quenching) effect and severe interference of excitation light scattering with emission signal extraction in complex physiological environments. It eliminates the self-absorption effect and endogenous background fluorescence interference, maximizes the effective signal, and significantly improves the sensitivity and signal-to-noise ratio of in vivo fluorescence detection. Attached Figure Description

[0023] Figure 1 The diagram shows the optimal emission and optimal excitation spectra of carbon dots;

[0024] Figure 2 The UV-Vis spectrum of carbon dots;

[0025] Figure 3 Fourier infrared spectrum of carbon dots;

[0026] Figure 4 The image shows a Raman spectrum, revealing two characteristic scattering bands typical of carbon materials. Detailed Implementation

[0027] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0028] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0029] Example 1

[0030] This embodiment provides a method for preparing red carbon dots with long Stokes shift, the specific steps of which are as follows:

[0031] (1) Weigh 0.3 g of p-phenylenediamine and place it in a stainless steel high-pressure reactor with a volume of 100 mL and a polytetrafluoroethylene liner. Add 30 mL of anhydrous ethanol and sonicate for 30 minutes to fully dissolve and uniformly disperse the precursor.

[0032] (2) The sealed reactor was placed in a homogeneous heating box and subjected to a solvothermal reaction at a constant temperature of 200℃ for 6 hours.

[0033] (3) After the reaction is complete, allow the reaction vessel to cool naturally to room temperature, collect the reaction liquid, and concentrate it under reduced pressure using a rotary evaporator;

[0034] (4) Transfer the concentrate to a dialysis bag with a molecular weight cutoff of 500 Da, and dialyze it in deionized water for 48 hours, changing the dialysis fluid every 8 hours during the process.

[0035] (5) The dialysis solution was freeze-dried to obtain reddish-brown solid powder carbon dots.

[0036] The properties of the obtained carbon dots were characterized:

[0037] Figure 1 The optimal emission and excitation spectra of carbon dots show that the optimal emission wavelength of carbon dots is 612 nm, which reaches the red light region; the Stokes shift reaches 144 nm, which can effectively overcome the problems that existed before.

[0038] Figure 2 This is the UV-Vis spectrum of carbon dots. The strong absorption peaks near 238 nm and 279 nm are due to the π-π bonds in the C=C bonds of the aromatic ring structure. * The absorption characteristics induced by electronic transitions. This also confirms that the precursor p-phenylenediamine did indeed undergo deep dehydration condensation and carbonization during the solvothermal process, successfully constructing a sp... 2 Hybridized conjugated carbon nuclei. Furthermore, a low-energy absorption peak appears in the spectrum near 516 nm. This peak is typically attributed to n-π surface states containing heteroatoms (such as CN, C=N, etc.). * Electron transition.

[0039] Figure 3 The image shows the Fourier transform infrared (FTIR) spectrum of the carbon dots. The CS and CN absorption signals in the FTIR spectrum confirm the successful retention of nitrogen and effective doping of sulfur in the system. Abundant N and S improve the water solubility of the carbon dots and provide dense nonradiative relaxation channels for electronic transitions, which is one of the reasons for their large Stokes shift of 144 nm. Furthermore, the 1400-1600 cm⁻¹... -1A broad characteristic absorption band appeared in the range, which is a typical vibrational mode of the aromatic C=C skeleton. This strong signal of the conjugated benzene ring structure is consistent with the conclusions of the ultraviolet spectroscopy, further confirming the large-size delocalized π-electron network of carbon (one of the reasons for red light emission).

[0040] Figure 4 The Raman spectrum shows two characteristic scattering bands of carbon materials, located at 1350 cm⁻¹. −1 The nearby defect band (D peak) represents sp 3 Hybridized carbon and disordered structure, and also located at 1580 cm⁻¹ −1 The nearby graphite band (G peak) represents sp 2 The in-plane stretching vibrations of hybrid carbon. The peak intensity of the G peak is significantly stronger than that of the D peak, thus forming highly regular and rigid graphitized nuclei within it. This highly crystalline sp... 2 The carbon framework gives the carbon dots good structural stability, providing structural support for the efficient emission of red light.

[0041] The above results indicate that the carbon dots prepared in this embodiment possess red light emission characteristics (612 nm) and a large Stokes shift (144 nm), and their structure contains sp... 2 Hybridized conjugated carbon cores, nitrogen-sulfur doped surface states, and highly ordered graphitized crystal nuclei meet the key performance requirements of red light probes for bioimaging applications.

Claims

1. A method for preparing red carbon dots with long Stokes shift, characterized in that, Includes the following steps: Step (1): Mix p-phenylenediamine and anhydrous ethanol at a mass-to-volume ratio of 0.01-0.02 g / mL, and sonicate to obtain a homogeneous mixture; Step (2): Place the homogeneous mixture in a high-pressure reactor and carry out a solvothermal reaction at 180-220℃ for 4-8 hours; Step (3): After the reaction is completed, cool to room temperature, collect the reaction solution, concentrate the reaction solution under reduced pressure, place the concentrate in a dialysis bag and dialyze it in deionized water for purification, and finally freeze-dry to obtain red carbon dots.

2. The preparation method according to claim 1, characterized in that, The ultrasonic treatment time in step (1) is 30 minutes.

3. The preparation method according to claim 1, characterized in that, In step (2), the homogeneous mixture is placed in a high-pressure reactor and subjected to a solvothermal reaction at 200°C for 6 hours.

4. The preparation method according to claim 1, characterized in that, The dialysis bag used in step (3) has a molecular weight cutoff of 500 Da, the dialysis time is 48 hours, and the dialysis solution is changed every 8 hours.

5. The preparation method according to claim 1, characterized in that, The mass ratio of the p-phenylenediamine precursor to the volume of anhydrous ethanol in step (1) is 0.3 g : 30 mL.

6. A red-light carbon dot, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 5.

7. The red-light carbon dot according to claim 6, characterized in that, The fluorescence emission wavelength of the red carbon dot is 612 nm, and the Stokes shift is 144 nm.

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