Laser-induced nucleation of urea through the control of Insoluble Impurity | Scientific Reports

Scientific Reports volume 14, Article number: 25777 (2024) Cite this article

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The mechanism of non-photochemical laser-induced nucleation (NPLIN) was investigated through the crystallization of urea crystals in supersaturated (151%) aqueous solutions. To detect the role of impurities in NPLIN, the effect of filtration, doping and laser irradiation position on nucleation probability were studied, respectively. As particles larger than the filter pore size in the solution were almost removed by syringe filter with poly (ether sulfone) membrane, the NPLIN probability was evidently suppressed by filtration when the pore sizes of filter were used from 1 μm to 0.45 μm. The inhibition of NPLIN after filtration could be reversed to varying degrees by adding several kinds and concentrations of impurities into the filtered solution; as the solid particles were extra added, age after cooling for samples was no longer necessary. The increase in the NPLIN probability with the irradiation height decreases was observed when samples were irradiated at different vertical positions within the sample vial. The experimental results were analyzed with reference to the known mechanisms. As nucleation probability through NPLIN can be controlled by pore size of syringe filter, doping or laser irradiation position, several functions were established to analyze the nucleation probability distribution under the effect of these three factors. These experimental results were discussed with reference to the known mechanisms proposed for NPLIN.

Crystallization of substances from solution or melt is a basic physicochemical process. The crystallization process is divided into two steps, nucleation and crystal growth. Based on several advantages such as multiple control effects, high spatial selectivity, wide application of materials and barely thermal damage to samples, non-photochemical laser induced nucleation (NPLIN) emerged as alternative to the traditional techniques1,2,3 and several assisted crystallization methods such as mechanical shock4,5,6,7, electric fields8,9, shear fields10, ultrasonic11,12,13,14, and microwave15,16,17.

Ever since NPLIN was discovered by Garetz et al.,18 accidentally in supersaturated urea solution, a series of studies were carried out in many different solutions, such as urea19, glycine20,21,22,23, halide salts24,25,26,27, proteins28 and organic small molecules29. NPLIN can also be used in supersaturated solution of volatile solute, such as carbon dioxide aqueous solutions30,31. Despite some theoretical and experimental literature were reported on NPLIN, the potential molecular-scale mechanism is uncertain. The discovery, phenomenology, and underlying mechanism of NPLIN were described systematically by Alexander et al.;32 the observations and existing literature are summarized and discussed in a recent review article by Korede et al.33.

Three major mechanisms have been proposed to explain the experimental observations separately. In a work of supersaturated urea solution by Garetz et al.,18 a remarkable phenomenon was observed: the growth direction of the initial needle-like urea crystal induced by laser was consistent with the angle of polarisation of the incident laser beam. A mechanism for the direction of the oscillating electric field of the light acting on molecules was proposed, which was based optical Kerr effect (OKE). In a study on supersaturated glycine solution, Garetz’s team20 further proved the feasibility to control the obtained polymorph via laser polarization, which was termed polarization switching. Agarwal and Peters34 recently summarized several unresolved issues with the OKE mechanism. The laser power threshold in the experiment is not readily explained; at low laser power, the polarization interaction energy is much lower than the background thermal energy, but it can also induce nucleation. In addition, Liu et al.35 found that there was no significant correlation between urea crystal angle and direction of linear polarization, and Irimia et al. showed that polarization switching was difficult to reproduce36.

The dielectric polarization (DP) model was proposed by Alexander and Camp, assumes that the electric field from the laser pulse polarizes and stabilizes subcritical solute clusters, such that clusters become critical and nucleate37. In a work of Hua et al.37. , microfluidic NPLIN device was designed to characterize crystals in actual time and high-throughput in situ. Their proposed two-step nucleation DP model predicted the decrease of the second free energy barrier. Knott et al.30 conducted a study of NPLIN of supersaturated carbon dioxide solutions, for gaseous solutes, DP model have little influence, while laser-induced carbon dioxide bubble nucleation has been observed, casting doubt on the applicability of the DP model.

Nanoparticle heating (NPH) has been proposed to explain some previously unexplained observations and several features of the NPLIN experiments32. In the NPH mechanism31, nanoscale cavity is formed when laser irradiation is absorbed by insoluble nanoparticle impurities in solution, which acts as sites for heterogeneous nucleation. According to NPH mechanism33, the nanoparticles absorb the laser power they are rapidly heated, which leads to the formation of vapor bubbles, nucleation may occurs at new gas-liquid interface or at locations of increased solute concentration; bubble collapses after expanding and outgoing pressure waves, which can lead to the rupture of the nanoparticles, nucleation may be induced at the location of bubble rupture and impurity rupture. The current evidence shows that the feasibility of NPLIN is highly dependent on the interaction between impurity particles and laser, rather than the intrinsic effect caused by the electric fields from laser pulse acting on solute molecules.

In attempting to understand the mechanism of NPLIN, various parameters and experimental conditions such as supersaturation36,38, laser intensity, exposure volume26, irradiation location26, filtration39, and impurities40 have been involved. Ward et al.40 studied the effect of impurities in laser-induced nucleation of ammonium chloride. In their work, the composition of impurities and the particle size distributions were analyzed, after doping similar impurities, the probability of NPLIN and the number of crystals recovered to the level of unfiltered. Briard et al.41 investigated the NPLIN phenomenon of ethylenediamine sulfate, they noted that filtration drastically reduces the NPLIN effect and systematically investigated the effect of doping with several metal particles with different sizes and optical properties on NPLIN efficiency, finding that the enhancement of the nucleation probability is related to the presence of insoluble particles dispersed within the aqueous solutions. Korede et al.27 designed a droplet-based microfluidic platform, and the effect of laser intensity, wavelength, supersaturation filtration and nanoparticles on the nucleation probability was quantified, their observations highlight the role of impurities and strengthen the NPH mechanism.

In the present paper, large size particles are removed directly from supersaturated urea solutions by syringe filters with different pore sizes, to study the effect of impurity size on NPLIN. The role of composition and particle size distribution of intentionally doped impurities is studied. The effect of irradiation position (vertical position within the sample vial) on the NPLIN probability of supersaturated urea solutions is observed.

The impurities added in the experiment were copper (II) phthalocyanine (CuPc) (90%, Maclin C806159-100 g, LOT C13250917), boron carbide (CB4) (99.5%, Maclin B828274-50 g, LOT C13738900) and silicon dioxide (SiO2) (99.99%, Maclin S817558-100 g, LOT C12644687). In order to control the number of doped solid nanoparticles, these three impurities and ultrapure water (18.2 MΩ cm) were used to prepare aqueous dispersions with mass fractions of 0.5 wt% and 2.0 wt%, respectively. Dispersion was placed in an ultrasonic bath for a period of 1 h, and shaken well before adding into the sample.

Supersaturated urea aqueous solution was used for experiments. Crystal growth of urea occur in a short time after laser-induced nucleation, which allows it easy to distinguish versus spontaneous nucleation. Concentrations are expressed as the number of moles of solute per kilogram of solvent (mol kg− 1). The saturation concentration (Csat) of urea solution at 30 ℃ is 22.48 mol kg− 142. Urea (99%, Maclin U8020349-500 g, LOT C13811737) and ultrapure water were used to prepare supersaturated urea solution. An aqueous urea solution (C) with a concentration of 34.04 mol kg− 1 was prepared with the supersaturation (S) of 1.51 at 30 ℃ (\(\:S=\frac{C}{{C}_{sat}}=\:1.51\)).

The solutions were placed in an oven (50 °C) for 8 h to ensure complete dissolution. The warm solutions (~ 4.5mL) were then transferred into pre-cleaned glass sample vials (5mL, Pyrex, 1.7 cm diameter, plastic screw-on caps with rubber inserts) by syringe, as shown in Fig. 1a. The containers and capacities of each experiment are given in Table S1 of the Supporting Information. All samples were reheated for 1.5 h inside an oven (50 ℃) to dissolve any spontaneous nucleation events, then placed in an incubator (30 ℃) for 3 h and cooled to 30 ℃. Filtered samples were prepared by filtering heated solution through syringe filters (Jin Teng, 0.45 μm, 0.60 μm, 0.80 μm, 1.00 μm, 1.50 μm, 2.00 μm poly (ether sulfone) (PES)). Vials and syringes were prewarmed by heating in an oven (50 ℃) for half an hour, but filters were not. Filter was replaced after filtering every 30 mL solution to remove any trace chemicals and spontaneous crystal on the filter membrane.

Solutions were passed through syringe filters with pore size of 0.45 μm and then impurities were added to prepare samples, which were called doped samples. Impurities are added before cooling to prevent high spontaneous nucleation probabilities. The mass of solution and aqueous dispersion of impurity in each doped sample is approximately 4.70 g and 0.034 g, respectively. Considering the ultrapure water in the aqueous dispersion, supersaturation of doped sample was reduced to 1.48, which had little effect on the nucleation probability. The mass fractions of impurity in the sample are listed in Table 1. Some samples undergo aging before being exposed to laser irradiation, aging involves incubating samples at 30 °C for the stated period.

The experimental design consisted of four sets of experiments. The influence of impurities on NPLIN was studied in many aspects. Experiment 1 consists of four groups of samples, which were used to study the effect of filtration on nucleation. Six sets of samples were prepared by four kinds of different syringe filters with different pore sizes (0.45 μm, 0.60 μm, 0.80 μm, 1.00 μm, 1.50 μm, 2.00 μm pore). Unfiltered, irradiated samples were used for reference. The samples were aged for 72 h before laser irradiation.

In the Experiment 2, 0.5 wt% and 2.0 wt% CuPc were added as impurities, respectively; the aging time was controlled within 0 ~ 14 days, the effects of different dispersion concentration and aging period were studied. In the Experiment 3, CB4 and SiO2 were added as impurities. The dispersion was prepared according to the method in Sect. Aqueous dispersion of impurity. If crystals were observed during aging period, the sample was removed from the dataset. As the impurities were added before cooling, it diffused for 3 h in solutions (samples without aging).

In Experiment 4, samples were prepared in glass test tubes (18.0 cm length, 1.9 cm diameter, plastic screw-on caps with rubber inserts) (as shown in the Fig. 1b) using the same procedure. In order to study the effect of laser irradiation position on nucleation probability, we divided the tube into three positions according to their height: low position (2.5 cm), middle position (10.5 cm) and high position (15.5 cm), as shown in Fig. 1b. Three groups of NPLIN experiments were conducted at these locations, and each group of experiments were performed for at least 20 samples, and aged for 48 h before NPLIN work.

(a) The picture of the sample vial used in Experiments 1, 2, and 3. The solution was loaded to a height of 2.4 cm. (b) Schematic diagram of laser irradiation position in Experiment 4. The high, middle or low positions were three laser irradiation positions, and the solution was loaded to a height of 17 cm.

The laser pulse was obtained from a Q-switched Nd3+: YAG laser (pulse duration: 6.0 ns, wavelength: 532 nm, repetition rate:10 Hz), and a circular pinhole was initially used to assist the direction of laser light, and it was removed once the direction of light was ensured. The beam was passed through a Glan-laser polarizer to control the transmitted power and to ensure purity of the linear polarization. The final diameter of the beam was shrunk to 1.0 mm by a variable beam reducer. Each vial was exposed to laser pulses for up to 30 s to induce nucleation, the irradiation was stopped immediately after the production of urea needle crystals was observed. Two different vials were used in experiment 1 and 4, their laser powers used were 1.55 mJ pulse− 1, and their power densities at the exit of vial were calculated around 75.98 MW cm− 2, which is detailed in Section S3 of Supporting Information; as the addition of impurities, the solution was prone to laser induced nucleation, the laser powers used in experiment 2 and experiment 3 were 0.50 mJ pulse− 1 with 25.4 MW cm− 2 energy density. These laser power values have taken into account the focusing effect of the vial acting as a powerful cylindrical lens. The experiments were carried out in supersaturated urea solution at 30 ℃, and the parameters of all experiments are detailed in Table S2 in the Supporting Information.

Particle size measurement, ultraviolet–visible spectrum and heat capacity were used to gain information about the properties of impurities. The particle size distributions of original impurities (CuPc, CB4, SiO2) were measured through a laser particle size analyzer (Omec LS-609). The three original impurities were mixed with ultrapure water, respectively, then dispersed in an ultrasonic cleaner for 1 h, particle size distribution was measured in a laser particle size analyzer using water as the medium. The particle size distributions of suspension population and bottom population of supersaturated urea solutions doped with 0.5 wt% CuPc (without aged and aged for 24 h) were also measured. The suspension population and bottom population were collected from at least 20 samples by dropper. The drops from supersaturated solutions are collected using a warm dropper and then deposited into a known quantity of water to prevent crystallization. Particle size analysis is performed using water as the medium, so the solutions will not crystallize during measurement. Frequency curves of particle size distribution were plotted based on the particle size distribution data. The absorbance of the three impurities (CuPc, CB4 and SiO2) was measured by ultraviolet–visible spectrometer (UV-VIS) (PerkinElmer, Lambda 950), the wavelength range is 450 to 550 nm. The heat flows of three impurities (CuPc, CB4, and SiO2) at various temperatures were measured by Differential Scanning Calorimeter (DSC) (PerkinElmer DSC 8000), the heating rate is 10 °C min− 1, the relationship between specific heat capacity and heat flow is simply determined by the mass of the substance, the temperature change, and the time.

In the Experiment 1, the filter condition was set as the unique variable, filtered samples were prepared by filtering solution through syringe filters. Six different filters in our experiments were chosen: syringe filters with poly (ether sulfone) membrane (0.45 μm, 0.60 μm, 0.80 μm, 1.00 μm, 1.50 μm, 2.00 μm pore). The nucleation probability of each batch of samples was counted during this experiment, as shown in Table 2.

Aging for the unfiltered supersaturated urea solution is necessary, and fresh urea solution did not nucleate. The solutions without aging (aging 0 h) were used immediately after cooling to 30 ℃ (3 h) in incubator, and no crystal was formed by NPLIN. The unfiltered samples were aged for 72 h and achieved a high nucleation probability after laser irradiation, 31 out of 41 samples crystallized within 30 s, the nucleation probability is 75.6%, detailed data are summarized in Table 2. As we expected, the filtration of urea solution caused a significant reduction in the nucleation probability through NPLIN, after passing through syringe filters with pore sizes of 2.00 μm, 1.50 μm, 1.00 μm, 0.80 μm and 0.60 μm, the nucleation probability was75%, 74%, 48.8%, 28.0% and 10.0% respectively. It was noteworthy that NPLIN was not observed when the pore of syringe filter was 0.45 μm, no crystals occurred. The results show that the nucleation probability of NPLIN decreases with decreasing filter pore size. Thus, filtration pore size can effectively control NPLIN probability.

As described in Sect. Effect of filtration, NPLIN probability can be effectively reduced by removing some large particles in the solution through filtration, which may be native impurities, external dust, or colloidal-scale solute structures. In the following part of the experiment, filtered samples were initially filtered through syringe filter with pore size of 0.45 μm, large particles in the original solution were removed as much as possible. After that, three kinds of different impurities were selected as additives to observe whether nucleation probability of NPLIN could be restored.

Aqueous dispersion of CuPc with mass fractions of 0.5 wt% and 2.0 wt% were used to dope solutions, and the results of NPLIN are shown in Table 3. It was found that the nucleation probability of the samples with 0.5 wt% CuPc significantly increased, with the NPLIN probability of the samples without aging being 100%. As the aging time increased, the NPLIN probability decreased and stabilized at 40.0% after 336 h, which was close to the probability observed at 72 h of aging. In the series of samples doped with 2.0 wt% CuPc, the NPLIN probability of the samples without aging was also 100.0%. With increased aging time, the nucleation probability decreased to 23.3%. The change of CuPc with the time scale was given in Fig. S1 and S2 of the Supporting Information.

The results showed that the effect of filtration on NPLIN could be reversed by adding impurities to the solution. However, to support this conclusion, more experimental data are needed. Further experiments were conducted to exploring the effects of different impurity (size, kind and concentration) on the nucleation probability.

The NPLIN probabilities for samples doped with CB4 and SiO2 are summarized in Table 4. In the presence of CB4 (0.5 wt% or 2.0 wt% CB4), all the samples without aging and aged for 24 h were nucleated though laser irradiation. However, With the addition of SiO2, no laser-induced nucleation event was observed in the samples without aging; after 24 h of aging, the NPLIN probabilities for samples doped with 0.5 wt% SiO2 and 2.0 wt% SiO2 are 4.0% (2 out of 50 samples were crystallized) and 16.7% (7 out of 42 samples were crystallized), respectively.

When the low position (2.5 cm) of the long tube sample was exposed to single pulses of laser, urea crystals were observed in the bottom area within a few seconds, the nucleation probability was 95.0%. After the middle position (10.5 cm) was irradiated by laser, the crystals appeared in the middle area of the tube, but some samples did not crystallize after 30 s of laser irradiation, and the nucleation probability was 66.7%. In the high position (15.5 cm), the crystal appeared in the upper part of the test tube, and the NPLIN probability was 25.0%, detailed data are summarized in Table 5. Urea crystals appeared near the laser irradiation position, which proves that the nucleation of urea was induced by laser, and the NPLIN probability decreased significantly with the increased of irradiation position height, which was inconsistent with the results reported by Korede et al.26 in potassium chloride solutions.

In this work, the size of particles in the solution was controlled by filtering the supersaturated urea solution through syringe filters with different pore sizes, the mechanism of laser-induced nucleation and the effect of particle size were investigated. The results show that particles smaller than 0.45 μm may be difficult to promote NPLIN in this experiment conditions (aging 72 h, laser power 1.55 mJ pulse− 1). This is consistent with the conclusion of Javid et al.,39 impurities removed by filtration are critical to the mechanism of laser-induced nucleation. In addition, after the solutions were filtered by the syringe filters with pore size of 0.60 μm, 0.80 μm and 1.00 μm, the laser still induced nucleation, but the nucleation probability was significantly reduced. This further shows that only particles with a certain size can affect the NPLIN activity in the filtered solution40,41.

Plot of the nucleation probability against the pore size of syringe filter after laser irradiation for equivalent supersaturation (1.51) samples which were aged for 72 h. Solid circles represent data for filtered solution. Wilson score was calculated as statistical uncertainty at 95% level of confidence interval (black error bar)43. Logistic growth function fitting to the data is shown as black solid lines. The area between the two dotted blue lines represents the range of particle sizes that can promote the NPLIN probability.

In the experiment of nucleating the filter solution by laser irradiation, the relationship between the nucleation probability f and the size d of the pore size of syringe filters is drawn in Fig. 2, detailed data are given in Table S3 of the Supporting Information. Referring to the values of filter pore size and nucleation probability in Table 2, the data points can be fitted with a logistic growth function:

where f(d) is the NPLIN probability at the pore size of syringe filters d; P is the NPLIN probability of the unfiltered sample (0.76). k is the growth rate of nucleation probability, which is 6.55; d0 is the midpoint of the function, the inflection point of the growth curve, which has a value of 0.90.

According to Fig. 2, as the syringe filter pore size increases, the nucleation probability also increases, eventually reaching the level observed in unfiltered samples. Specifically, when the pore size is less than 0.45 μm, nucleation is not induced by laser irradiation, indicating that particles below this size threshold are insufficient to promote NPLIN. The model described by Eq. 1 shows that the growth rate of nucleation probability is highest when the pore size is approximately 0.9 μm.

Our additional experiments with pore sizes of 1.5 μm and 2.0 μm further validate the logistic growth model, showing NPLIN probabilities of 0.74 and 0.75, respectively. These results closely align with the logistic model’s prediction, where the probability plateaus around 0.76. This suggests that the effective range of particle sizes promoting NPLIN extends beyond 1.6 μm, with particles as large as 2.0 μm continuing to significantly influence nucleation probability. It can be hypothesized that while larger particles in the solution contribute significantly to NPLIN, these particles may also accumulate and settle during the aging period. This settling process could lead to a decrease in the number of particles actively participating in nucleation within the laser pathway. As a result, the total number of effective particles capable of activating NPLIN does not continue to increase beyond a certain pore size threshold, such as 1.6 μm. Consequently, the nucleation probability remains unchanged, or plateaus, after filtering through these larger pore sizes, as shown by the stable NPLIN probabilities at 1.5 μm and 2.0 μm.

Under these experimental conditions, it can be considered that particles smaller than 0.45 μm cannot effectively promote NPLIN. Instead, nucleation probability is primarily dependent on particles larger than 0.45 μm. The implication of this broader size range is significant for understanding the mechanisms driving NPLIN, as it indicates that larger particles contribute more effectively than previously understood.

The new data reinforce the logistic model’s suitability for capturing the observed trends and offer a more nuanced perspective on the influence of particle size on NPLIN activity. While the model initially set an upper guideline limit at d = 1.6 μm, the data demonstrate that this should be seen as a guideline rather than a strict cutoff. The presence of particles larger than this size, as shown by the 2.0 μm data, suggests that a wider range, potentially extending well beyond 1.6 μm, is relevant for NPLIN activity.

These findings indicate that, although particles smaller than 0.45 μm are insufficient for NPLIN, particles up to and potentially beyond 2.0 μm remain effective. This understanding of how particle size distributions influence NPLIN has important implications for the study and application of this phenomenon. It also guides future research to further explore and delineate the size range that impacts NPLIN, which could lead to optimized conditions for controlling nucleation processes.

Bar plots showing the nucleation probability for different positions of the laser beam with respect to the interfaces within the glass test tubes. The nucleation probability is shown in parentheses above the corresponding bar. These samples were aged for 48 h, and the Wilson score was calculated as statistical uncertainty at 95% level of confidence interval (black error bar)43.

From the perspective of the NPH mechanism, locally increase the concentration of impurities promote NPLIN activity, which has been shown in previous experiments40. According to Sect. Effect of position, the NPLIN probability decreases with the laser irradiation height under the same conditions. This conclusion was verified in subsequent experiments. At 6.0 cm of the long test tube, 16 of 20 samples were nucleated by laser, with the probability of 80.0%; 10 out of 21 samples were nucleated at 13.0 cm of the long test tube, with the NPLIN probability of 46.7%. Figure 3 provides an overview of the correlated nucleation probability in the position experiments. An increased in the nucleation probability was observed when the position of the laser beam was reduced. These results indicated an indirect relationship between impurities and laser-induced nucleation, impurities tend to form an uneven distribution throughout the sample volume with the influence of aging time and gravity, the number of impurities in low position (2.5 cm) is the largest, and the impurities in the high position (15.5 cm) are the least.

It is worth noting that in a study recently reported by Korede et al.26 no correlation between NPLIN probability and irradiation position other than the air/ solution interface (meniscus) was found, the increase in NPLIN probability at meniscus was attributed to interface adsorption, laser refraction or dust/impurity particles adhere. This phenomenon could be attributed to several factors, including the dimensions of the sample vial used in their study, which were 6.10 cm × 1.66 cm. In their research, the volume of the solution in the sample was 7 mL, and the calculated height of the solution (H) was approximately 3.23 cm. The maximum distance difference in laser irradiation on the solution sample was 0.9 H, equating to 2.9 cm, which is significantly less than the distance difference in our experiments (15.50 cm). One of the reasons for no significant change in the nucleation probability in their experiments could be due to the too shorter distance of laser irradiation. Furthermore, Korede’s study involved a different system compared to ours; they researched inorganic molecules whilst our study focused on organic material, several studies indicate that organic solutes require several days of aging to make NPLIN feasible29,44,44,45,46,−47, whereas inorganic crystals can be induced without aging by NPLIN24,25,37,40,48. Thus, the inclusion of the aging factor makes it challenging to effectively analyze the correlation between inorganic and organic molecular systems of NPLIN. In NPLIN experiments with different heights in long tubes, the solutes and impurities continuously agglomerate during the aging period, and distributed with the effect of gravity and height32,33,40. Assuming an uneven distribution of impurities throughout the sample volume, and that the solution is a viscous fluid and the impurities are ideal spherical particles, the relationship between sedimentation rate and particle size may be estimated. The sedimentation rate is proportional to the particle size, see Section S5 of Supporting Information for details.

Organic systems such as glycine20,21,22,23, urea18,19, L-histidine47 and carbamazepine29 can be crystallized by NPLIN. It is important to note that aging is necessary in these systems29,44,45,46,−47 . Proper aging time could allow impurity particles with small sizes gathering together and forming sufficient sizes which could be nucleation sites. However, in the experiment, it was observed that after doping CuPc or CB4, laser can induce the nucleation of supersaturated urea solution without aging (aging 0 h). This may be because the initial size and quantity of the external impurities in this experiment are sufficient. According to Table S4 in the Supporting Information, the relationship between the aging time t and NPLIN probability f in solutions with CuPc is given. All fits use the following logistic decay function:

Where f(t) is the NPLIN probability when the aging time is t; Pmax is the maximum value of the probability, the value is 1; Pmin is the minimum value of the probability, which is 0.40 and 0.23 for 0.5 and 2.0 wt% CuPc, respectively. The parameters c and t0 depend upon the corresponding fitting of the data, c is the decay rate of nucleation probability, which is 1.56 and 1.59 d− 1 (day− 1) for 0.5 and 2.0 wt% CuPc, respectively.; t0 represents the midpoint of the function, that is, the inflection points of the curve, which is 1.78 and 1.49 d (day) for 0.5 and 2.0 wt% CuPc, respectively.

Plots of the nucleation probability against the aging time after laser irradiation for equivalent supersaturation (1.51) with different concentrations of CuPc in the samples. Black square and red circle represent data for samples with 0.5wt% CuPc and 2.0wt% CuPc respectively. Wilson score was calculated as statistical uncertainty at 95% level of confidence interval (black error bar and red error bar)43. Functions fitting to the data are shown as black curve (0.5 wt%) and red curve (2.0 wt%).

The NPLIN probability decreases with the extension of aging time as is shown in Fig. 4, the decay rate is the fastest when the aging time is about 1.78 d and 1.49 d for 0.5 and 2.0 wt% CuPc, respectively. The nucleation probability stabilizes after gradually decreasing to about 0.40 and 0.23, respectively.

It can be seen from Fig. 5 that suspended CuPc particles are easy to settle over time, which may be a reason why the nucleation probability decreases with increasing aging time. The particle size distributions of the suspended and bottom particle populations were measured for solutions doped with 0.5 wt% CuPc, which is detailed in Section S9 of Supporting information. As is illustrated in Fig. 5a, the particle size distribution of suspended particles in the samples without aging was wider, with 3.09 μm particles accounting for the largest proportion; after 24 h of aging, the particle size distribution was narrower, with 1.29 μm particles being the most abundant. The particle distribution of the bottom population can be found in Fig. 5b, compared to without aging, the particle size distribution was wider after aged for 24 h, with the largest proportion of particle sizes increasing from 5.07 μm to 6.49 μm. The results showed that large particles concentrated at the bottom, while small particles were suspended in the middle and upper part of the solution. After undergoing one day of aging, most of the solid particles larger than 1.29 μm settled at the bottom of the solution and did not interact with the laser. Compared to the 100% nucleation probability obtained with the non-aged sample, the 83.3% nucleation probability observed is not solely due to particles of one specific size, such as those around 1.29 μm, but rather due to the combined effect of a range of particle sizes. Figure 5a shows a distribution of particle sizes that collectively contribute to the nucleation process. This indicates that multiple sizes are involved, with larger particles potentially acting as nucleation sites. The reason the nucleation rate did not stay at 100% is that the large particles settled during aging, reducing both the quantity and size of the suspended particles in the solution. This reduction in suspended particles decreased the laser nucleation effect, as fewer particles were available to interact with the laser pulses effectively.

Particle size distribution frequency diagram of solutions doped with 0.5 wt% CuPc, (a) suspended population; (b): bottom population. The blue curve and red curve represent data for aged for 24 h and without aging, respectively. As impurities were added before the samples were cooled, impurities were diffused for 3 h in samples without aged.

In this study, three different kinds of impurities were added in supersaturated urea solutions to observe and compare their effects on nucleation probability. Compared with CuPc and CB4, it was observed that SiO2 could not significantly restore NPLIN activity. In order to interpret this observation, the particle size distributions of the three original impurities were measured, and the particle size distribution frequency curves are shown in Fig. 6a.

Particle size distributions of the original impurities (a): Particle size distribution frequency diagram. Blue curve, black curve and red curve represent data for CuPc, CB4 and SiO2, respectively. (b): Volume distribution of particle with size 0.45–1.60 μm. Blue column, black column and red column represent data for CuPc, CB4 and SiO2, respectively.

The average volume diameter of CB4, CuPc, and SiO2 are 2.73 μm, 7.25 μm, and 12.16 μm, respectively. At 0 h of aging, the nucleation probability order is CB4 = CuPc > SiO2, whereas at 24 h of aging, the nucleation probability order is CB4 > CuPc > SiO2. This is opposite to the average volume diameter ordering compared to the NPLIN probability ordering. In Sect. Effect of pore size of syringe filters on nucleation, according to Eq. 1, we observed that when the filter pore size reaches 1.5 μm and 2.0 μm, the laser-induced nucleation probabilities are 0.74 and 0.75, respectively. These values are consistent with the model’s prediction that the nucleation probability plateaus around 0.76. This empirical evidence confirms that for pore sizes exceeding 1.6 μm, the nucleation probability is similar to that of unfiltered samples, as particles larger than 1.6 μm continue to effectively contribute to NPLIN. It can be assumed that particles with sizes between 0.45 and 1.60 μm play a major role in NPLIN. According to Fig. 6b, it can be found that the particle concentration in this range is ranked as CB4 > CuPc > SiO2. However, it is important to note that particle concentration was measured without aging, whereas nucleation probability was measured after 24 h of aging, so these two datasets cannot be directly compared. Furthermore, the lower NPLIN activity of SiO2 may also be due to its lower light absorption compared to CB4 and CuPc, indicating that optical properties play an important role in the nucleation process. Additionally, the UV-Vis spectra (Supplemental Information S10) reveal significant differences in absorption characteristics among the impurities. As noted, SiO2 exhibits much lower absorption in the UV-Vis spectrum compared to CuPc and CB4, which significantly affects its NPLIN activity. The low absorbance is a key factor in its reduced nucleation probability, as absorption is critical for effective laser-induced nucleation.

In the solutions doped with SiO2, samples were irradiated by laser after cooling, the nucleation probability was 0. An experiment with laser irradiation on the bottom (about 0.5 cm) of these samples was performed for reference, the nucleation probabilities for samples doped with 0.5 wt% SiO2 and 2.0% SiO2 are 6.0% (3 out of 50 samples were nucleated) and 9.5% (4 out of 42 samples were nucleated), respectively. This may be due to the large volume diameter of SiO2 (8.32–19.30 μm), which allows the impurities to leave the laser irradiation region quickly and concentrate at the bottom. In addition, with the aging time of 24 h, a small number of samples doped with SiO2 were nucleated by laser, this result may be caused by the existence of few small sized impurities in the aqueous dispersion of impurity (laser particle size analyzer Omec LS-609 cannot identify particles with distribution less than 1%). During aging period, small impurity particles will agglomerate to form large particles, the interaction of these particles with the laser may have promoted nucleation, heterogeneous nucleation caused by these impurities may also be a reason.

Alexander et al. suggested that the NPLIN mechanism based on the OKE and electric field caused by laser pulses cannot explain several experimental features37, but it can be reasonably explained by the effect of impurities. Impurities provide a way to transfer laser energy into solution, explaining the presence of threshold laser power. By controlling the pore size of the syringe filter, resulting some impurities are removed, NPLIN is inhibited to varying degrees. Type and mass fraction of impurities explain the differences in the results of NPLIN studies in different systems. Aging effects in Organic systems may be explained by time-dependent aggregation of small particles. As things stand, the NPLIN results for urea can be explained by a mechanism for NPLIN based on heating of impurity particles40. An insoluble impurity particle is rapidly heated to high temperatures by laser pulses, resulting in formation of vapor bubble; a vapor bubble expands and collapses, possibly resulting in the formation of nuclei at fresh interfaces. The details of the particle-heating mechanism are shown in Fig. 7.

Referring to the work of Alexander et al.37 schematic diagrams of the proposed mechanism for NPLIN based on impurity particle heating. (a) Particles in solution in the path of the laser beam are irradiated. (b) A portion of the particles absorb the energy of laser pulses and are rapidly heated to high temperatures. (c) The heat transferred to the solution results in the formation of expanding vapor bubbles of varying sizes. (d) Collapse and break of vapor bubbles. (e) Rupture of larger bubbles may lead to the induction of nucleation at new interfaces. (f) Crystal induced by laser growth to a certain size.

For the underlying mechanism that different impurities lead to changes in NPLIN probabilities, previous studies have demonstrated the significant impact of particle size and optical properties on NPLIN efficiency. In the NPH mechanism, the bubbles created when particles absorb laser energy and are heated are the key factors. As SiO2 has the largest average volume diameter among the three original additives, it is the most difficult to heat quickly to high temperatures. Additionally, the optical properties of the particles must be considered; colorless and transparent SiO2 particles exhibit low optical absorption compared to CuPc and CB4. The ultraviolet–visible spectrums from 450 to 550 nm of SiO2, CuPc, and CB4 from were given in the Supporting Information (Figure S3). The absorbances of SiO2, CuPc, and CB4 at 532 nm were 0.04, 0.73 and 0.86. Specifically, small black particles of CB4 lead to high optical absorption and heating rates, while colorless transparent and large size particles of SiO2 result in low optical absorption and heating rates. In the supplementary information section S11, the heat capacities of the three impurities further explain that lower heat absorption causes SiO2 to have a lower nucleation rate.

It is important to recognize the foundational work by Briard et al.41, as well as other earlier studies40, which explored the influence of solid impurities on the NPLIN mechanism. Briard et al.41 reported that the presence of insoluble particles in supersaturated solutions is essential for NPLIN efficiency. Their research demonstrated that filtering these solutions significantly reduces the NPLIN effect, underscoring the critical role of such impurities in efficient nucleation. Additionally, the study revealed that the particle size and optical properties of the impurity particles are pivotal in determining NPLIN activity. These experimental observations provide strong support for the impurity-heating mechanism, where impurity particles absorbing laser pulses and forming bubbles must be considered. The findings align with previous research, confirming that both particle size and optical properties are vital factors influencing NPLIN probabilities.

In summary, in NPLIN experiments on supersaturated urea solution, impurities play a crucial role. As syringe filter pore size, doping, aging, and height were related to impurities, the effect of these factors on nucleation probability were investigated. Laser-induced nucleation was greatly suppressed by filtration, and the nucleation probability could be controlled by changing the pore size of syringe filter. NPLIN activity in filtered solutions was restored by doping with solid impurity particles, our analysis reveals that different kinds of impurities influence the enhancement of NPLIN probability through their optical properties rather than just their size. In particular, the low NPLIN probability of SiO2 can be attributed to its optical properties, such as low light absorption, which is critical in laser-induced nucleation processes. This finding emphasizes the importance of considering both size and optical characteristics when evaluating impurity effects on NPLIN. It is noteworthy that the necessary aging time for nucleation in urea solutions was eliminated after the addition of solid impurities. For samples with different irradiation positions, nucleation probability increased as the laser irradiation height decreased. Functions based on the data of nucleation probability versus pore size and aging time were constructed to analyze NPLIN results. Our study provides results that support previous works on the NPLIN mechanism, which propose that the presence of impurity particles is crucial for nucleation. The evidence from our experiments aligns with established theories on nanoparticle heating and the role of optical properties in enhancing nucleation probability. Factors such as the size, aggregation, and sedimentation of impurity particles are crucial in influencing NPLIN efficiency. Our findings contribute to the growing body of evidence supporting the critical influence of impurities in laser-induced nucleation processes. We hope that the discussion and results presented in this paper further contribute to understanding and validating a comprehensive NPLIN mechanism.

Data is provided within the manuscript or supplementary information files.

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The authors gratefully acknowledge the National Natural Science Foundation of China (grant Nos. 52162002, and 22366016), and Jiangxi Provincial Natural Science Foundation (No. 20232BAB213035) their generous support of this work, and cordially thank the Jiangxi University of Science and Technology for providing various characterization facilities.

The authors gratefully acknowledge the National Natural Science Foundation of China (grant Nos. 52162002, and 22366016), and Jiangxi Provincial Natural Science Foundation (No. 20232BAB213035) their generous support of this work.

Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou, 341000, People’s Republic of China

Shuai Li, Xiongfei Xie, Qingqing Qiu & Yao Liu

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Shuai Li and Xiongfei Xie contributed equally to this work, regarding the implementation of the research and analysis of the results. Yao Liu and Qingqing Qiu contributed to the design of the research and the writing of manuscript. Yao Liu conceived the original and supervised the project. All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

Correspondence to Yao Liu.

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Li, S., Xie, X., Qiu, Q. et al. Laser-induced nucleation of urea through the control of Insoluble Impurity. Sci Rep 14, 25777 (2024). https://doi.org/10.1038/s41598-024-77557-6

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Received: 24 April 2024

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DOI: https://doi.org/10.1038/s41598-024-77557-6

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