Owing to the flow configuration, two distinct materials termed "Class I particles", and "Class II particles" were prepared, depending on the injection position of the chemical precursors (Figure , 1e, S2 and Table ). Class I particles were synthesized using a total flow rate (TFR, sum of all flow rates) of 750 μL/min and a FRR of 4 by injecting an ethanolic solution of Fe(BF4)2 (0.4 M) in the side inlet channels at a flow rate of 300 μL/min and an aqueous solution of Htrz (2M) in the central inlet channel at a flow rate of 150 μL/min. Meanwhile, Class II particles were generated keeping the TFR and FRR constant but switching the injection position of the precursors. Thus, the Htrz solution was injected into the side inlet channels at a flow rate of 300 μL/min, while the Fe(BF4)2 solution was introduced through the central inlet channel at a flow rate of 150 μL/min. By changing the position of the reagents while keeping the other parameters (TFR and FRR) constant, we expected to see important physicochemical differences in the formation of the [Fe(Htrz)2(trz)]BF4 particles owing to the different concentration profiles and radial diffusion of the chemical precursors inside the microfluidic device (Figure and). The TFR and FRR values used in these syntheses were optimized to grow the RD zone within the main microfluidic channel as fast as possible to maximize the reaction time (Figure ), while simultaneously enabling the large-scale production of SCO particles at industrially relevant quantities (≈ 8 g/day in a single device, Table ). It is important to note that too low TFR and FRR can cause clogging of the microfluidic device, whereas too high TFR and FRR will shorten the RD zone (Figure ), reducing the continuous production of SCO particles and the time available for the proper stoichiometric formation of the SCO complex (see SI, Section S2 and Table ). Regardless of the class of the particles being formed, the [Fe(Htrz)2(trz)]BF4 materials were isolated from the microfluidic device as solid particles with a purple hue. We evaluated the morphology and size of Class I and II particles using scanning electron microscopy (SEM). Clear morphological differences become apparent when comparing the two configurations: Class I particles exhibit a rod-like shape with an average diameter of 293 nm, whereas Class II particles display both spherical and elongated shapes with a smaller average size of 195 nm (Figure ). To explain these differences, it is essential to consider several aspects. First, the diffusion coefficient of Fe(II) is approximately half that of Htrz (Table ). This means that, regardless of the FRR and precursor injection position and FRR (central or side), Htrz will diffuse throughout the entire cross-section of the channel, while Fe(II), due to its lower diffusion coefficient, will only do so when it acts as the focusing flow or when it is weakly focused (see numerical simulation results in SI, Section S2 and Figure ). The second key factor is that the flow configuration controls the relative amounts of each reagent introduced. For instance, at a FRR of 4, the molar flow (flow rate multiplied by concentration) of Fe(II) injected into the channel differs drastically from one class to the other -0.00024 mol/min in Class I versus 0.00006 mol/min in Class II, i.e. a 4-fold difference. Thus, depending on the conditions, Fe(II) may be either in excess or in deficit of Htrz (and vice versa), relative to the stoichiometric 1:3 Fe:Htrz ratio required for the formation of the SCO complex. Furthermore, taking the aspects of diffusion and stoichiometry into account, the diffusion of precursors within the main channel can still result in a non-stoichiometric distribution within the RD zone even with the precursor solutions injected at the correct stoichiometric ratio, potentially hindering the proper formation of SCO particles if Fe(II) does not diffuse sufficiently. Based on these considerations and the SEM images, we conclude the following: in Class I particle synthesis, an excess of Fe(II) in the main channel, along with an RD zone that spans the entire channel (Figure , S25 and S27), facilitates continuous particle formation and larger particle growth. In contrast, Class II particle formation involves introducing a twenty-fold excess of Htrz (the focusing flow or sheath flow) relative to Fe(II) into the channel. This configuration creates an RD zone confined to the region where Fe(II) diffuses (Figure , S26 and S27). Although the restricted RD zone theoretically facilitates stoichiometric balance within its boundaries, the significantly low availability of Fe(II) ultimately limits particle growth. As a result, the particles formed in Class II are smaller than those in Class I. In terms of morphology, an excess of Fe(II) in Class I appears to promote the formation of rod-shaped particles, whereas a deficiency in Fe(II) leads to incomplete rod shaping, resulting in rounded particles. To the best of our knowledge, rod-shaped structures are the preferred morphology during [Fe(Htrz)2(trz)]BF4 synthesis across various synthetic techniques, although nanocubes can also be achieved under specific conditions. Thus, we attribute the incomplete shaping in Class II particles to the insufficient Fe(II) availability.