Morphologically Diverse Micro- and Macrostructures Created via Solvent Evaporation-Induced Assembly of Fluorescent Spherical Particles in the Presence of Polyethylene Glycol Derivatives

The creation of fluorescent micro- and macrostructures with the desired morphologies and sizes is of considerable importance due to their intrinsic functions and performance. However, it is still challenging to modulate the morphology of fluorescent organic materials and to obtain insight into the factors governing the morphological evolution. We present a facile bottom-up approach to constructing diverse micro- and macrostructures by connecting fluorescent spherical particles (SPs), which are generated via the spherical assembly of photoisomerizable azobenzene-based propeller-shaped chromophores, only with the help of commercially available polyethylene glycol (PEG) derivatives. Without any extra additives, solvent evaporation created a slow morphological evolution of the SPs from short linear chains (with a length of a few micrometers) to larger, interconnected networks and sheet structures (ranging from tens to >100 µm) at the air–liquid interface. Their morphologies and sizes were significantly dependent on the fraction and length of the PEG. Our experimental results suggest that noncovalent interactions (such as hydrophobic forces and hydrogen bonding) between the amphiphilic PEG chains and the relatively hydrophobic SPs were weak in aqueous solutions, but play a crucial role in creating the morphologically diverse micro- and macrostructures. Moreover, short-term irradiation with visible light caused fast morphological crumpling and fluorescence switching of the obtained structures.

Han et al. recently demonstrated red fluorescent spherical particles (SPs) and 1D fibrous structures generated via the self-assembly of a new type of aggregation-induced emission enhancement (AIEE [52][53][54][55][56])-active chromophores with different terminal functional groups [57,58]. Nevertheless, it is still challenging to achieve the facile growth of such fluorescent SP building blocks into diverse micro-and macrostructured materials (from micrometer-sized chains and necklaces to macrometer-sized interconnected network structures) and to obtain insight into the determinants for governing the morphological evolution.
Polyethylene glycol (PEG), which contains a repeating unit of -(CH2CH2O)n-, possesses the following characteristics: (i) It dissolves in water, as well as in commonly used organic solvents. (ii) The ethylene unit and oxygen in the PEG chain can occasionally show amphiphilic characteristics that have hydrophobicity and hydrophilicity [59][60][61][62][63]. (iii) It does not aggregate in a dilute aqueous solution [64,65]. (iv) It does not interfere with the spherical assembly of AIEE-active propeller-shaped chromophores (Bu, Figure 1). (v) It has wide-ranging chemical and biomedical applications [59][60][61][66][67][68][69]. In this study, we chose commercially available polyethylene glycol (PEG) derivatives to link SPs for these five reasons. Here, we describe a simple bottom-up approach to creating diverse microand macrostructures via the solvent evaporation-induced assembly of Bu SP building blocks with the help of PEG chains ( Figure 1). As the PEG fraction and the PEG length increased, the SPs connected faster and generated micrometer-sized linear and branched chains. They then linked together to develop into macrometer-sized interconnected networks and sheet structures, as the solvent evaporated further. In addition, we also investigated their visible-light-triggered morphological crumpling.

Growth of Organic SP Building Blocks into Diverse Micro-and Macrostructures
Whereas flexible PEG does not aggregate in a dilute aqueous solution, photoisomerizable (C 3 -symmetric→asymmetric conformation changes) Bu has a strong tendency to assemble into fluorescent SPs [58], and the resultant SPs are well dispersed in THF-H 2 O Molecules 2021, 26, 4294 3 of 11 mixed solutions. Therefore, we hypothesized that if a linear PEG chain does not interfere with the spherical assembly of Bu in a PEG:Bu binary mixed solution, the SPs and water-soluble PEGs would move independently in dilute solutions. However, slow solvent evaporation would improve the frequency of effective collision between SPs and PEG chains. As a result, the relatively hydrophobic SPs would be connected by linear PEG with amphiphilic characteristics through hydrophobic interactions [59][60][61][62][63] to evolve into larger, interconnected structures ( Figure 1).
To test our hypothesis, we first changed the mPEG1000 concentration (from 1 to 50 mg/L, in H 2 O) at a fixed Bu concentration (50 µM = 50 mg/L, in THF), followed by varying the mixing ratio (v/v) of mPEG1000:Bu (mPEG1000 fraction = f mPEG1000 , %) and the molecular weight of PEG. According to our preliminary experimental results, the spherical assembly of Bu was not hindered by the coexistence with PEG chains and provided fluorescent SPs with diameters of~50-500 nm. In addition, the PEG fraction had an important role in connecting SPs. For instance, in the initial stage with a small fraction of PEG (f mPEG1000 = 9%), individual SPs underwent random motion without conspicuous flocculation. However, as the solvents evaporated, the SPs very slowly connected to produce short linear chains containing ≤5 spheres ( Figure 2b). Slightly longer chains and partially branched chains, which are composed of approximately ≤20 SPs, were frequently produced from a sample containing a PEG fraction of 50% ( Figure 2c and Supplementary Materials Figure S1). The inset scanning electron microscopy (SEM) image in Figure 2c confirms that organic SPs forming such chains roughly retained their original spherical shape and were nested inside pea-like frames, which were presumably composed of PEG chains.

Growth of Organic SP Building Blocks into Diverse Micro-and Macrostructures
Whereas flexible PEG does not aggregate in a dilute aqueous solution, photoisomer izable (C3-symmetric→asymmetric conformation changes) Bu has a strong tendency t assemble into fluorescent SPs [58], and the resultant SPs are well dispersed in THF-H2O mixed solutions. Therefore, we hypothesized that if a linear PEG chain does not interfer with the spherical assembly of Bu in a PEG:Bu binary mixed solution, the SPs and water soluble PEGs would move independently in dilute solutions. However, slow solven evaporation would improve the frequency of effective collision between SPs and PEG chains. As a result, the relatively hydrophobic SPs would be connected by linear PEG wit amphiphilic characteristics through hydrophobic interactions [59][60][61][62][63] to evolve int larger, interconnected structures ( Figure 1).
To test our hypothesis, we first changed the mPEG1000 concentration (from 1 to 5 mg/L, in H2O) at a fixed Bu concentration (50 µM = 50 mg/L, in THF), followed by varyin the mixing ratio (v/v) of mPEG1000:Bu (mPEG1000 fraction = fmPEG1000, %) and the molecula weight of PEG. According to our preliminary experimental results, the spherical assembl of Bu was not hindered by the coexistence with PEG chains and provided fluorescent SP with diameters of ~50-500 nm. In addition, the PEG fraction had an important role in con necting SPs. For instance, in the initial stage with a small fraction of PEG (fmPEG1000 = 9% individual SPs underwent random motion without conspicuous flocculation. However, a the solvents evaporated, the SPs very slowly connected to produce short linear chains con taining ≤5 spheres ( Figure 2b). Slightly longer chains and partially branched chains, whic are composed of approximately ≤20 SPs, were frequently produced from a sample contain ing a PEG fraction of 50% ( Figure 2c and Supplementary Materials Figure S1). The inse scanning electron microscopy (SEM) image in Figure 2c confirms that organic SPs formin such chains roughly retained their original spherical shape and were nested inside pea-lik frames, which were presumably composed of PEG chains. In contrast, at higher PEG fractions, such as 66, 80, and 86%, longer branched chain were often observed together with interconnected networks formed by merging man In contrast, at higher PEG fractions, such as 66%, 80%, and 86%, longer branched chains were often observed together with interconnected networks formed by merging many necklace-like structures that were mostly 2-10 µm in size (Figures 2d and 3). Notably, the inset SEM image in Figure 2d shows the existence of thread-like streaks around the network structures, suggesting that the interconnected structures were buttressed by the PEG shell. The inner SPs in the shells often turned into an oval or short rod shape, which is likely due to the long-term interactions between soft SPs and long PEG chains at the air-water interface. necklace-like structures that were mostly 2-10 µm in size (Figures 2d and 3). Notably, the inset SEM image in Figure 2d shows the existence of thread-like streaks around the network structures, suggesting that the interconnected structures were buttressed by the PEG shell. The inner SPs in the shells often turned into an oval or short rod shape, which is likely due to the long-term interactions between soft SPs and long PEG chains at the airwater interface.

Control Experiments: Need of PEG Chains for Morphological Growth
We conducted controlled experiments to validate the need for PEG for the morphological evolution into larger, more intricate microstructures. The importance of the presence of PEG was clearly verified at the border where solvents evaporated. In the absence of PEG, fast-moving SPs behaved independently and were heaped up to form random mounds on a hydrophilic glass substrate (Movie S1 and Figure 2a). In sharp contrast, in the presence of PEG, ready-made interconnected structures were stacked sideways to produce denser structures (Movie S2).

In Situ Morphological Evolution Processes at Higher PEG Fractions
To visualize the morphological evolution process, we next carried out in situ OM observations of mPEG1000:Bu binary mixed systems (fmPEG1000 = 66 and 86%) by gradually evaporating the solvents. Upon incubation for ~20 min under our experimental conditions (22-23 °C and 50-55% humidity, Figure 3a), the spheres started to slowly hook up to virtually invisible things considered to be mPEG1000, resulting in short chains. As the solvent evaporated further, the short chains linked together to form longer chains and branched chains (Figure 3b). In the case of the mPEG1000:Bu binary mixed system with fmPEG1000 = 66%, the long branched chains and round necklace-like structures were connected to one another to construct giant interconnected structures larger than 100 µm (Figure 3c,d). Importantly, once the SPs were hooked up to invisible PEG chains, the resultant intricate structures did not separate but rather floated like a single group at the air-water interface (Movie S3). These results support our earlier hypothesis that weak noncovalent interactions between the relatively hydrophobic SPs and the amphiphilic PEG chains play a crucial role in evolving into diverse micro-and macrostructures.

Control Experiments: Need of PEG Chains for Morphological Growth
We conducted controlled experiments to validate the need for PEG for the morphological evolution into larger, more intricate microstructures. The importance of the presence of PEG was clearly verified at the border where solvents evaporated. In the absence of PEG, fast-moving SPs behaved independently and were heaped up to form random mounds on a hydrophilic glass substrate (Movie S1 and Figure 2a). In sharp contrast, in the presence of PEG, ready-made interconnected structures were stacked sideways to produce denser structures (Movie S2).

In Situ Morphological Evolution Processes at Higher PEG Fractions
To visualize the morphological evolution process, we next carried out in situ OM observations of mPEG1000:Bu binary mixed systems (f mPEG1000 = 66 and 86%) by gradually evaporating the solvents. Upon incubation for~20 min under our experimental conditions (22-23 • C and 50-55% humidity, Figure 3a), the spheres started to slowly hook up to virtually invisible things considered to be mPEG1000, resulting in short chains. As the solvent evaporated further, the short chains linked together to form longer chains and branched chains (Figure 3b). In the case of the mPEG1000:Bu binary mixed system with f mPEG1000 = 66%, the long branched chains and round necklace-like structures were connected to one another to construct giant interconnected structures larger than 100 µm (Figure 3c,d). Importantly, once the SPs were hooked up to invisible PEG chains, the resultant intricate structures did not separate but rather floated like a single group at the air-water interface (Movie S3). These results support our earlier hypothesis that weak noncovalent interactions between the relatively hydrophobic SPs and the amphiphilic PEG chains play a crucial role in evolving into diverse micro-and macrostructures.
Moreover, when the mPEG1000 fraction increased up to 86% (Figure 3e-h) and the molecular weight of the PEG increased to 2000 (Figure 4 and Figure S2), SPs connected faster to form tortuous long chains and branched chains in the early stage, which, in turn, grew into dendrimer-like structures with many branches. As the solvent evaporated further, the large dendrimers stuck together and gradually developed into a giant mesh-like structure (Figure 5a, Figure S3). In the meantime, SPs became densely packed, as indicated by the yellow arrows in Figure 3h. Further solvent evaporation caused the mesh-like networks to be stacked horizontally on a hydrophilic glass substrate, occasionally producing a red fluorescent sheet exceeding 100 µm in size (Figure 5b).
Moreover, when the mPEG1000 fraction increased up to 86% (Figures 3e-h) molecular weight of the PEG increased to 2000 (Figure 4 and Figure S2), SPs con faster to form tortuous long chains and branched chains in the early stage, which, grew into dendrimer-like structures with many branches. As the solvent evapora ther, the large dendrimers stuck together and gradually developed into a giant me structure (Figure 5a, Figure S3). In the meantime, SPs became densely packed, as in by the yellow arrows in Figure 3h. Further solvent evaporation caused the mesh-l works to be stacked horizontally on a hydrophilic glass substrate, occasionally pro a red fluorescent sheet exceeding 100 µm in size (Figure 5b).   Figure 6 shows UV-vis absorption and Fourier-transform infrared (FT-IR) spe Bu dilute solution displays three characteristic absorption bands at 265, 380, and (Figure 6a), which are likely due to the short-axis Φ-Φ* transition [70], the π-π* tra of the azobenzene unit, and the combined effect of an intramolecular proton-tran action (keto-hydrazone form) and the energetic proximity of the (π,π*) and (n,π* [71][72][73][74][75][76][77], respectively. As the spherical assembly of Bu molecules and subsequen assisted morphological evolution proceeded, the three absorption bands became b and red-shifted to >275, 396, and >515 nm, respectively. Moreover, when the mPEG1000 fraction increased up to 86% (Figures 3e-h) and the molecular weight of the PEG increased to 2000 (Figure 4 and Figure S2), SPs connected faster to form tortuous long chains and branched chains in the early stage, which, in turn, grew into dendrimer-like structures with many branches. As the solvent evaporated further, the large dendrimers stuck together and gradually developed into a giant mesh-like structure (Figure 5a, Figure S3). In the meantime, SPs became densely packed, as indicated by the yellow arrows in Figure 3h. Further solvent evaporation caused the mesh-like networks to be stacked horizontally on a hydrophilic glass substrate, occasionally producing a red fluorescent sheet exceeding 100 µm in size (Figure 5b).   Figure 6 shows UV-vis absorption and Fourier-transform infrared (FT-IR) spectra. A Bu dilute solution displays three characteristic absorption bands at 265, 380, and 506 nm (Figure 6a), which are likely due to the short-axis Φ-Φ* transition [70], the π-π* transition of the azobenzene unit, and the combined effect of an intramolecular proton-transfer reaction (keto-hydrazone form) and the energetic proximity of the (π,π*) and (n,π*) states [71][72][73][74][75][76][77], respectively. As the spherical assembly of Bu molecules and subsequent PEGassisted morphological evolution proceeded, the three absorption bands became broader and red-shifted to >275, 396, and >515 nm, respectively.  Figure 6 shows UV-vis absorption and Fourier-transform infrared (FT-IR) spectra. A Bu dilute solution displays three characteristic absorption bands at 265, 380, and 506 nm (Figure 6a), which are likely due to the short-axis Φ-Φ* transition [70], the π-π* transition of the azobenzene unit, and the combined effect of an intramolecular proton-transfer reaction (keto-hydrazone form) and the energetic proximity of the (π,π*) and (n,π*) states [71][72][73][74][75][76][77], respectively. As the spherical assembly of Bu molecules and subsequent PEG-assisted morphological evolution proceeded, the three absorption bands became broader and red-shifted to >275, 396, and >515 nm, respectively. Molecules 2021, 26, x FOR PEER REVIEW 6 of 11 In addition, unlike a Bu SP sample in the absence of PEG, a quite broad band in the range of 3600-3100 cm −1 and strong bands in the range of 2950-2800 cm −1 emerged in the IR spectrum for fully dried interconnected structures (Figure 6b), which are attributable to hydrogen-bonded O-H and sp 3 C-H stretching vibrations mainly originating from PEG chains, respectively. By comparison, the phenyl-hydrogen stretching mode at 3020 cm −1 and the aromatic C-C stretching vibrations at 1601, 1484, and 1469 cm −1 , which originate from Bu chromophores, considerably weakened. We interpreted these experimental results as follows. As the PEG fraction increases, compared to the terminal -OH group, the number of -(CH2CH2O)n-units increases enormously. Therefore, the hydrophobic ethylene units are likely to be frequently exposed to the hydrophobic parts of Bu SPs through hydrophobic interactions [59][60][61][62][63]. By contrast, the hydrophilic adjacent oxygen and -OH group seem to be mainly directed toward water and form hydrogen bonds with water molecules. Such noncovalent interactions between PEG chains and SPs are weak but not inescapable. That is, an amphiphilic PEG chain (i) acts as an important linker connecting the individual SPs in the early stage of solvent evaporation and (ii) subsequently helps the resulting shorter chains evolve into larger, interconnected micro-and macrostructures.

Light-Sensitive Interconnected Structures
Irradiation of AIEE-active Bu with visible light leads to changes in the UV-vis absorption spectra almost identical to those of 365 nm light irradiation ( Figure S4) [58]. Notably, 1 H NMR data measured after exposure to sunlight indicate that C3-symmet-ric→asymmetric conformation changes are caused by light in the region from ultraviolet to visible light ( Figure S4c). Hence, we expected that if the Bu SPs were indeed nested in a pea-like PEG frame or shell with low melting points [78], the obtained diverse microand macrostructures would undergo discernible morphological deformation originating from light-induced conformation changes of Bu chromophores.
To check our assumption, we exposed samples to visible light (405 and 436 nm). First, compared to the laser confocal microscopy (LCM) image obtained by the first measurement under light illumination (λex = 405 nm), the image obtained by the second measurement revealed that the color of the chains became lighter, and the width of the chains became about two or more times wider (Figure 7a,b). That is, it was almost impossible to observe the intact interconnected network structures composed of PEG and SPs with LCM In addition, unlike a Bu SP sample in the absence of PEG, a quite broad band in the range of 3600-3100 cm −1 and strong bands in the range of 2950-2800 cm −1 emerged in the IR spectrum for fully dried interconnected structures (Figure 6b), which are attributable to hydrogen-bonded O-H and sp 3 C-H stretching vibrations mainly originating from PEG chains, respectively. By comparison, the phenyl-hydrogen stretching mode at 3020 cm −1 and the aromatic C-C stretching vibrations at 1601, 1484, and 1469 cm −1 , which originate from Bu chromophores, considerably weakened. We interpreted these experimental results as follows. As the PEG fraction increases, compared to the terminal -OH group, the number of -(CH 2 CH 2 O) n -units increases enormously. Therefore, the hydrophobic ethylene units are likely to be frequently exposed to the hydrophobic parts of Bu SPs through hydrophobic interactions [59][60][61][62][63]. By contrast, the hydrophilic adjacent oxygen and -OH group seem to be mainly directed toward water and form hydrogen bonds with water molecules. Such noncovalent interactions between PEG chains and SPs are weak but not inescapable. That is, an amphiphilic PEG chain (i) acts as an important linker connecting the individual SPs in the early stage of solvent evaporation and (ii) subsequently helps the resulting shorter chains evolve into larger, interconnected micro-and macrostructures.

Light-Sensitive Interconnected Structures
Irradiation of AIEE-active Bu with visible light leads to changes in the UV-vis absorption spectra almost identical to those of 365 nm light irradiation ( Figure S4) [58]. Notably, 1 H NMR data measured after exposure to sunlight indicate that C 3 -symmetric→asymmetric conformation changes are caused by light in the region from ultraviolet to visible light ( Figure S4c). Hence, we expected that if the Bu SPs were indeed nested in a pea-like PEG frame or shell with low melting points [78], the obtained diverse micro-and macrostructures would undergo discernible morphological deformation originating from light-induced conformation changes of Bu chromophores.
To check our assumption, we exposed samples to visible light (405 and 436 nm). First, compared to the laser confocal microscopy (LCM) image obtained by the first measurement under light illumination (λ ex = 405 nm), the image obtained by the second measurement revealed that the color of the chains became lighter, and the width of the chains became about two or more times wider (Figure 7a,b). That is, it was almost impossible to observe the intact interconnected network structures composed of PEG and SPs with LCM because of the light-induced morphological deformation being too fast. This was in sharp contrast to the Bu SPs, which had a very slow light response, as clearly shown in the LCM image in Figure S5.
Molecules 2021, 26, x FOR PEER REVIEW 7 because of the light-induced morphological deformation being too fast. This was in s contrast to the Bu SPs, which had a very slow light response, as clearly shown in the L image in Figure S5. Secondly, to lower the sphere-to-isotropic phase transition rate of SPs in the shells, we chose a low-intensity blue light source (~1-2 mW/cm 2 , 436 nm). The SEM im taken after short-term irradiation with blue light revealed that the center of the par marked by a white arrow was dented, and its rounded edge still remained (Figure 7 Upon sufficient exposure to blue light for 30 min (Figure 7e), all the particles insid PEG shell fully melted, and the round shapes disappeared completely. Eventually, the wrinkled PEG shells remained on the glass surface.
Moreover, when exposed to green light (520-550 nm) attached to the fluoresc optical microscopy (FOM), the interconnected network structures with AIEE charact tics began to melt within one second, and their red fluorescence switched off within (Movie S4 and Figure S6). The light response speed of the PEG:Bu binary mixed sys was ~10 times faster than that without PEG. The fast light responses were due to component assembly systems [79] consisting of both PEG derivatives with low me points and fluorescent SPs with a light-induced sphere-to-isotropic phase transition.

Conclusions
Fluorescent organic micro-and macrostructures were readily formed using the P assisted assembly of soft SPs building blocks at the air-liquid interface. Our experim results revealed that their morphologies and sizes can be readily modulated from li chains and branched chains (with a size of a few micrometers) to giant dendrimer structures, interconnected networks, and sheets (ranging from tens to >100 µm in size slow solvent evaporation. At an early stage, the amphiphilic PEG chain served as an portant linker connecting the fluorescent SPs and subsequently had the long-term i actions with the SPs to create giant interconnected structures. Eventually, the PEG supported the fluorescent micro-and macrostructures. In addition, fast visible-lightgered morphological crumpling and fluorescence intensity changes were success substantiated through OM, FOM, LCM, SEM, and in situ OM observations. These fin will be useful for mimicking stimuli-responsive biological systems found in nature. Secondly, to lower the sphere-to-isotropic phase transition rate of SPs in the PEG shells, we chose a low-intensity blue light source (~1-2 mW/cm 2 , 436 nm). The SEM image taken after short-term irradiation with blue light revealed that the center of the particle marked by a white arrow was dented, and its rounded edge still remained (Figure 7c,d). Upon sufficient exposure to blue light for 30 min (Figure 7e), all the particles inside the PEG shell fully melted, and the round shapes disappeared completely. Eventually, only the wrinkled PEG shells remained on the glass surface.
Moreover, when exposed to green light (520-550 nm) attached to the fluorescence optical microscopy (FOM), the interconnected network structures with AIEE characteristics began to melt within one second, and their red fluorescence switched off within 3-5 s (Movie S4 and Figure S6). The light response speed of the PEG:Bu binary mixed systems was~10 times faster than that without PEG. The fast light responses were due to twocomponent assembly systems [79] consisting of both PEG derivatives with low melting points and fluorescent SPs with a light-induced sphere-to-isotropic phase transition.

Conclusions
Fluorescent organic micro-and macrostructures were readily formed using the PEGassisted assembly of soft SPs building blocks at the air-liquid interface. Our experimental results revealed that their morphologies and sizes can be readily modulated from linear chains and branched chains (with a size of a few micrometers) to giant dendrimer-like structures, interconnected networks, and sheets (ranging from tens to >100 µm in size) via slow solvent evaporation. At an early stage, the amphiphilic PEG chain served as an important linker connecting the fluorescent SPs and subsequently had the long-term interactions with the SPs to create giant interconnected structures. Eventually, the PEG shell supported the fluorescent micro-and macrostructures. In addition, fast visible-lighttriggered morphological crumpling and fluorescence intensity changes were successfully substantiated through OM, FOM, LCM, SEM, and in situ OM observations. These finding will be useful for mimicking stimuli-responsive biological systems found in nature.

Materials
Tetrahydrofuran (THF, spectroscopic grade, Kanto Kagaku, Japan) was chosen as a good solvent to dissolve the Bu molecule. Polyethylene glycol monomethyl ether 1000 (mPEG1000, average molecular weight (Mw) of 950-1050) and polyethylene glycol 2000 (PEG2000, Mw = 1900-2100) were purchased from Tokyo Chemical Industry Co., Ltd. Bu, whose molecular weight (Mw = 1003) is almost identical to mPEG1000, was prepared according to the literature [13b]. Ultrapure water (which was purified to reach a minimum resistivity of 18.0 MΩ·cm (25 • C) using a µPure HIQ water purification system, Romax, South Korea) was used for all experiments.

PEG-Mediated SP Assembly into Various Microstructures
mPEG1000 H 2 O solutions in the concentration range of 1-50 mg/L were added dropwise, under mild shaking, into a Bu THF solution (50 µM = 50 mg/L), respectively. The resulting turbid suspension did not precipitate for at least 2-3 days but was well maintained until the PEG fraction (by volume) reached~86% and over. After the suspension was aged in a volumetric flask for about 20 min,~100 µL of the mixed suspension was carefully placed onto a clean glass or quartz substrate. To minimize unexpected side effects such as a sudden fluctuation in the solvent evaporation and the resulting change in the aggregation rate, all the experiments were conducted under the same experimental conditions (22-23 • C and 50-55% humidity).

Characterization
Optical microscopy (OM), fluorescence optical microscopy (FOM, λ ex = 520-550 nm), and laser confocal microscopy (LCM) images were taken using an Olympus BX53 microscope and LEXT OLS4000 3D laser microscope (λ ex = 405 nm) after placing a few drops of the PEG:Bu SP mixed suspension onto a clean glass or quartz substrate. The FE-SEM (field-emission scanning electron microscopy: HITACHI SU8020 and TESKAN-MIRA3-LM) samples were coated with an approximately 5-10 nm-thick platinum layer using a Cressington 108 auto sputter coater, Ted Pella, Inc. The transmission electron microscopy (TEM) was performed at 120 kV using a JEOL JEM-1400 Plus. UV-vis absorption and fluorescence spectra were recorded using a Shimadzu UV-2600 UV-vis spectrophotometer and a Horiba FluoroMax-4 spectrofluorometer, respectively. Fourier-transform infrared (FT-IR) spectra were recorded on a PerkinElmer (spectrum 100) spectrometer. Samples were exposed to light (Tokina Supercure-204S, generated by a combination of Toshiba color filters) to investigate their light response.
Supplementary Materials: The following are available online. Figure S1: SEM image of the sample obtained from the mPEG1000:Bu mixed system (f mPEG1000 = 50%), Figure S2: TEM and SEM images showing various microstructures formed by connecting the SPs through PEG2000, Figure S3: SEM images of mesh-like structures, Figure S4: 1 H NMR, absorption, and fluorescence spectra, Figure S5: LCM image of relatively photostable Bu spheres, Figure S6: Visible-light-triggered morphological deformation and rapid fluorescence intensity response, Movie S1: A control experiment (mp4), Movie S2: In the presence of PEG, ready-made interconnected structures were stacked sideways (mp4), Movie S3: In situ OM observation of a giant interconnected necklace structure (mp4), Movie S4: Fast light responses: In situ FOM of mesh-like structures observed during visible light irradiation for 5 s (mp4).