## 1 Introduction  
Microbial single-cell biosensors have become valuable tools in metabolic engineering due to their easy detection through a fluorescent output signal, their single-cell resolution and their compatibility with viable cells [1,2]. Biosensors facilitate the screening or selection process for a desired product by sensing the presence of the inconspicuous molecule coupled to a conspicuous reporter [2]. These reporters are often based on transcription regulators or riboswitches, as these molecules undergo a conformational change triggered by binding of the analyte, which is directly linked to transcriptional control of the reporter gene [1,2]. Natural biosensors are rare because of the lack of known regulators that are specific for the detection of a desired metabolite. Therefore, intense efforts have been undertaken in biosensor identification and characterization in order to monitor intracellular concentrations of industrially relevant compounds. Rational strain engineering for industrial purposes is often limited by the high complexity of metabolic networks. On the other side, classical strain development based on random mutagenesis is typically limited by the screening capacity, and often lacks an easy to manage readout system to judge the performance of the generated mutants [3]. Biosensors allow high-throughput screening for the monitoring of the production performance, at least for products accumulating intracellularly, while production performance for secreted products can only be deduced indirectly [1,4,5]. Biosensors have been shown to augment and accelerate metabolic engineering based on a new build-test-learn cycle [5]. Isoprenoid pyrophosphates such as GGPP are typically present in low concentrations in the cell. Therefore, an effective biosensor is sought for terpenoid process/strain optimization. Isoprenoid pyrophosphates are building blocks for the synthesis of many high-value terpenoids, including the carotenoid astaxanthin or the sesquiterpenoid patchoulol [6, 7, 8]. These important secondary metabolites find various applications in the food, feed and cosmetic industries, either as additives or as high-performance ingredients in the health industry [9,10]. Chemical synthesis as well as isolation from natural sources is expensive and/or results in insufficient amounts, and thus the microbial production of terpenoids is receiving increasing attention [11,12]. As an example, the high-value astaxanthin is produced with the microalgae _Haematococcus pluvialis_[13], the red yeast _Xanthophyllomyces dendorhous_[14] and the bacterium _Paracooccus carotifiaciens_[15]. Microbial carotenoid production by engineered strains is on the rise, as much higher production titers, for example of 6.5 g/L \(\beta\)-carotene with optimized _Yarrovia lipolytica_, can be achieved [16]; however, the industrial production of carotenoids by engineered organisms is currently rare. Besides the intensive engineering of the precursor supply and the optimization of terminal carotenoid biosynthesis, central carbon fluxes as well as cofactor-regeneration might be promising targets [16] in order to achieve carotenoid production titers of \(>\)10 g/L. There are two engineered biosensors that exist pertaining to the detection of mevalonate, an intermediate of the mevalonate pathway of isopentenyl pyrophosphate (IPP) biosynthesis [17,18], which cannot be used to monitor IPP biosynthesis via the MEP pathway. Direct IPP sensing has been achieved by a synthetic fusion of the IPP-binding isopentenyl pyrophosphate:dimethyllylly pyrophosphate isomerase Idi with the DNA-binding domain of AraC [19]. This fluorescence reporter responded to the extracellular addition of mevalonate to an _E. coli_ strain equipped with the mevalonate pathway; however, no direct evidence was observed that intracellular IPP was sensed by the synthetic biosensor [19].